Prior to this MR there were two GitHub related importers:

* Github::Import: the main importer used for GitHub projects
* Gitlab::GithubImport: importer that's somewhat confusingly used for
  importing Gitea projects (apparently they have a compatible API)

This MR renames the Gitea importer to Gitlab::LegacyGithubImport and
introduces a new GitHub importer in the Gitlab::GithubImport namespace.
This new GitHub importer uses Sidekiq for importing multiple resources
in parallel, though it also has the ability to import data sequentially
should this be necessary.

The new code is spread across the following directories:

* lib/gitlab/github_import: this directory contains most of the importer
  code such as the classes used for importing resources.
* app/workers/gitlab/github_import: this directory contains the Sidekiq
  workers, most of which simply use the code from the directory above.
* app/workers/concerns/gitlab/github_import: this directory provides a
  few modules that are included in every GitHub importer worker.

== Stages

The import work is divided into separate stages, with each stage
importing a specific set of data. Stages will schedule the work that
needs to be performed, followed by scheduling a job for the
"AdvanceStageWorker" worker. This worker will periodically check if all
work is completed and schedule the next stage if this is the case. If
work is not yet completed this worker will reschedule itself.

Using this approach we don't have to block threads by calling `sleep()`,
as doing so for large projects could block the thread from doing any
work for many hours.

== Retrying Work

Workers will reschedule themselves whenever necessary. For example,
hitting the GitHub API's rate limit will result in jobs rescheduling
themselves. These jobs are not processed until the rate limit has been
reset.

== User Lookups

Part of the importing process involves looking up user details in the
GitHub API so we can map them to GitLab users. The old importer used
an in-memory cache, but this obviously doesn't work when the work is
spread across different threads.

The new importer uses a Redis cache and makes sure we only perform
API/database calls if absolutely necessary.  Frequently used keys are
refreshed, and lookup misses are also cached; removing the need for
performing API/database calls if we know we don't have the data we're
looking for.

== Performance & Models

The new importer in various places uses raw INSERT statements (as
generated by `Gitlab::Database.bulk_insert`) instead of using Rails
models. This allows us to bypass any validations and callbacks,
drastically reducing the number of SQL queries and Gitaly RPC calls
necessary to import projects.

To ensure the code produces valid data the corresponding tests check if
the produced rows are valid according to the model validation rules.

The GitHub importer (and probably other parts of our code) ends up
calling Feature.persisted? many times (via Gitaly). By storing this data
in RequestStore we can save ourselves _a lot_ of database queries.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/39361

This adds the keyword argument "return_ids" to
Gitlab::Database.bulk_insert. When set to `true` (and PostgreSQL is
used) this method will return an Array of the IDs of the inserted rows,
otherwise it will return an empty Array.

By using SQL::Union we can return a proper ActiveRecord::Relation,
making it possible to select the columns we're interested in (instead of
all of them).

Add Ingress to cluster applications section

See merge request gitlab-org/gitlab-ce!15185

Upgrade vendored templates for 10.2

See merge request gitlab-org/gitlab-ce!15259

Improve GitLab Import rake task to work with Hashed Storage and Subgroups

Closes #36509

See merge request gitlab-org/gitlab-ce!15157

Fix merge_request_spec

See merge request gitlab-org/gitlab-ce!15250

Add support for k8s cluster applications - Helm tiller installation

See merge request gitlab-org/gitlab-ce!14908

Revert Auto DevOps gear animation

Closes #39798

See merge request gitlab-org/gitlab-ce!15232

Merge branch '38464-k8s-apps' of https://gitlab.com/gitlab-org/gitlab-ce into add-ingress-to-cluster-applications

Fixed multi-file editor table cells content width jumping

See merge request gitlab-org/gitlab-ce!15245

Merge branch '38464-k8s-apps' of https://gitlab.com/gitlab-org/gitlab-ce into add-ingress-to-cluster-applications

Make restoring backup database table warning more explicit

Closes #39903

See merge request gitlab-org/gitlab-ce!15246

Fix commit pipeline showing wrong status

Closes #39878

See merge request gitlab-org/gitlab-ce!15238

Fix spec. spec/serializers/cluster_application_entity_spec.rb and spec/serializers/cluster_entity_spec.rb

don't re-run smart interval callback if there is already one in progress

Closes #38394

See merge request gitlab-org/gitlab-ce!15032

Fix arguments error on Import/Export fetch_ref method

Closes #39541

See merge request gitlab-org/gitlab-ce!15241

issue ordering docs

Closes #39712

See merge request gitlab-org/gitlab-ce!15243

Refactor issuables index actions

See merge request gitlab-org/gitlab-ce!15242

Resolve "Warning icon in jobs dropdown has incorrect style"

Closes #39831

See merge request gitlab-org/gitlab-ce!15221