Drop Go v1.12 support

See merge request gitlab-org/gitlab-workhorse!532

We officially use Go v1.13+ for everything
(https://docs.gitlab.com/ee/install/requirements.html#go-versions), and
dropping Go v1.12 allows us to use new methods such as
`http.NewRequestWithContext`.

Document changelog as part of process

See merge request gitlab-org/gitlab-workhorse!530

Generating a changelog entry is now required when a merge request
warrants a CHANGELOG entry.

[ci skip]

Read and parse LSIF references

See merge request gitlab-org/gitlab-workhorse!524

In order to be able to "Find references" of a given range or
definition, we need to be able to read `references` items from
the LSIF file.

This will then be written to the JSON file for each range via
`references` property.

The property will look something like:

```
"references": [
  {
    "path": "main.go#L7"
  }
]
```

Each reference will be an object with `path` property. The `path`
property will point to the exact line where it is being used.

This is currently behind a feature flag (`code_navigation_references`)
which will set `ProcessLsifReferences` header to `true` when
enabled. If `false`, the `references` won't be read and generated.

Refactor to prepare for parsing references

See merge request gitlab-org/gitlab-workhorse!523

Eagerly delete private tempfiles

See merge request gitlab-org/gitlab-workhorse!526

Preparation for supporting other type of item (e.g. references).

This MR includes the following changes:
- Rename `RawDefRef` to `RawItem`.
- Rename `DefRef` to `Item`.

Remove deprecated option

See merge request gitlab-org/gitlab-workhorse!513

Add missing bounds checks in LSIF parser

See merge request gitlab-org/gitlab-workhorse!527

Make upload filter code (EXIF, LSIF) more uniform

See merge request gitlab-org/gitlab-workhorse!525

Log errors if lsif parser failed to close

See merge request gitlab-org/gitlab-workhorse!521

[ci skip]

Use latest Golang in order to build code navigation

See merge request gitlab-org/gitlab-workhorse!522

Cache hovers offsets in a file instead of a hash

See merge request gitlab-org/gitlab-workhorse!501

Storing them in a hash consumes a lot of RAM
Introducing a file helps to reduce RAM consumption significantly

Cache ranges value in a file instead of a hash

See merge request gitlab-org/gitlab-workhorse!500

Storing them in a hash consumes a lot of RAM
Introducing a file helps to reduce RAM consumption significantly

Use simplified setup for code intelligence

See merge request gitlab-org/gitlab-workhorse!520

Truncate hover values to 250 utf-8 characters

See merge request gitlab-org/gitlab-workhorse!511

Super long documentation is not very usable, but consumes a lot
of memory

[ci skip]

Send hover tokens instead of raw html

See merge request gitlab-org/gitlab-workhorse!517

Fix SendURL functionality when the upstream lacks Content-Type

Closes #267

See merge request gitlab-org/gitlab-workhorse!510

Two cases in particular, lacking a `Content-Type`,  gave us trouble:

* Transfer-Encoding: chunked
* No content-type and no transfer-encoding

Both of these are permitted by the HTTP RFC (cases 3 and 7), and we
can talk to arbitrary HTTP servers via sendurl, so it's imperative that
we handle them correctly. This commit adds tests for both cases.

Responses of the latter type are transparently converted to responses
of the former type. This is an automatic behaviour of the Go stdlib,
which doesn't really support making the second type of response
directly. Since Transfer-Encoding is a hop-by-hop header, this type of
encoding is extremely common, and we're still streaming, instead of
accumulating, the data, I think this is acceptable.

It's a more secure way to display documentation hovers
Rendering html is also a natural task for frontend