Display last commit of deleted branch in push events

## What does this MR do?

Display the last commit of a deleted branch in the push events of a project.

## Are there points in the code the reviewer needs to double check?

Is the change in `app/models/event.rb` the correct way to display a two-line event for deleted branches?

## Why was this MR needed?

It is easier to restore an accidentally deleted branch if the commit hash is displayed in the push events.

## What are the relevant issue numbers?

Fixes #18659

## Screenshots

### Before garbage collection

![before-gc](/uploads/5674cd53e1564d48b7f2f8406ea0fbed/before-gc.png)

### After garbage collection

![after-gc](/uploads/80950c1932feeb3b69d0fc11b8f7acf4/after-gc.png)

See merge request !4699

Fix encrypted data backwards compatibility after upgrading attr_encrypted gem

Adds missing attribute to attr_encrypted so it is fully backwards-compatible. Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19073

See merge request !4963

Add custom highlighting via .gitattributes

## What does this MR do?
Allows user control of language selection via a `gitlab-language` entry in `.gitattributes`

## Are there points in the code the reviewer needs to double check?
(paired with @stanhu)

## Why was this MR needed?
Guessing languages by filename is fraught and often wrong. In one project, `foo.pl` may be perl, and in another it may be prolog. Users might have a Thingfile that needs ruby highlighting, or depend on things that can't work in general, like `*.C` (capitalized) mapping to C++ instead of C.

This allows the user to override language choice so they never have to look at a mis-highlighted file.

## What are the relevant issue numbers?
https://github.com/jneen/rouge/issues/494
https://gitlab.com/gitlab-org/gitlab-ce/issues/13818 (*.tpl can't in general map to Smarty)
https://gitlab.com/gitlab-org/gitlab-ce/issues/13615 (in cases we don't have a language and mis-identify it, users could map to 'text' to turn off highlighting)

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added (N/A)
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4606