Port of 28458-present-gitlab-version-for-v4-changes-on-docs

See merge request !1285

Fix admin email notification recipient group select list

Closes #1656

See merge request !1277

Present GitLab version for each V3 to V4 API change on
v3_to_v4.md. And add Merge Request link to the v3 to v4
documentation. Similar to changelog, mention the Merge Request id &
link for each change.

Chat slash commands show labels correctly

See merge request !1280

[Elasticsearch] More efficient search

See merge request !1282

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Backport "Change branch_name param to branch throughout V4 API"

See merge request !1276

# Conflicts:
#   doc/api/v3_to_v4.md

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

Update Elasticsearch to 5.1

Closes #1253

See merge request !1059

API: Make subscription API more RESTfuL

See merge request !1274

Update documentation

Fix https://gitlab.com/gitlab-org/gitlab-ee/issues/1656

Add filtered search to MR page

See merge request !1243

Don't try to update a project's external service caches on a secondary Geo node

See merge request !1273

Squash default

See merge request !1272

git-annex may add a pre-commit hook on the GitLab server, which will
cause the commit to fail unless git-annex is configured. As there
shouldn't be one on the server normally, ignore all commit hooks.

For large repos, the checkout as part of creating a worktree can be very
expensive. This can be so expensive that processes get 'stuck' on
GitLab.com.

To make this cheaper, configure sparse checkout for squash worktrees so
that we only check out the files that were changed in the target
branch (new files don't need to be included).

Also add specs to cover the case where only new files are added, and the
case where the diff isn't shown in the merge request because it's too
large.

Fixes scheduling issue related to all mirrors

See merge request !1183

Include a string in all squash errors, and the reference of the MR that
is failing to squash.

The last commit is a bad default to use, because if you want to squash
commits, the last commit message will often be of the type 'fix
specs'. Instead, use the MR title and author as the commit's author.

Git-Annex to Git-LFS migration guide

Closes #1693

See merge request !1230

Added weight slash command

Closes #852

See merge request !1255

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