Based on review comment fetching labels hook_attrs is now
wrapped in an issue's model method.

We were not calling hook_attrs on the labels correctly. Specs were
passing because the issues under test did not have any labels!

Enables frozen string for the following:

* lib/gitlab/hook_data/**/*.rb
* lib/gitlab/i18n/**/*.rb
* lib/gitlab/import/**/*.rb
* lib/gitlab/import_export/**/*.rb
* lib/gitlab/kubernetes/**/*.rb
* lib/gitlab/legacy_github_import/**/*.rb
* lib/gitlab/manifest_import/**/*.rb
* lib/gitlab/metrics/**/*.rb
* lib/gitlab/middleware/**/*.rb

Partially addresses gitlab-org/gitlab-ce#47424.

This removes all usage of soft removals except for the "pending delete"
system implemented for projects. This in turn simplifies all the query
plans of the models that used soft removals. Since we don't really use
soft removals for anything useful there's no point in keeping it around.

This _does_ mean that hard removals of issues (which only admins can do
if I'm not mistaken) can influence the "iid" values, but that code is
broken to begin with. More on this (and how to fix it) can be found in
https://gitlab.com/gitlab-org/gitlab-ce/issues/31114.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/37447

Signed-off-by: Rémy Coutable <remy@rymai.me>

This adds various foreign key constraints, indexes, missing NOT NULL
constraints, and changes some column types from timestamp to
timestamptz.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31811

Signed-off-by: Rémy Coutable <remy@rymai.me>