Remove "subscribed" field from API responses returning list of issues or merge requests

Closes #22374

See merge request !9661

Add gitaly server dependency

Closes gitaly#109

See merge request !9722

Improve pipeline triggers API

See merge request !9713

In GitLab 9.0.0 we want to turn on Gitaly in Omnibus installations.
Gitaly is not a required component yet for source installations
because we want to iterate on the configuration and architecture some
more. Omnibus installations will have the option to completely disable
Gitaly if they want to or if necessary.

The new GITALY_SERVER_VERSION file which version of the Gitaly server
application is needed by the current GitLab version.

Remove remnants of git annex

See merge request !9508

Resolve "Improve Omnibus-Mattermost Command installation flow"

See merge request !9257

Delete artifacts for pages unless expiry date is specified

See merge request !9716

Fix json response in branches controller

Closes #28898

See merge request !9710

Use ETag to improve performance of issue notes polling

Closes #27582

See merge request !9036

Change language for testing colour fallback

See merge request !9537

Update profiles/account view to display new username

Closes #28655

See merge request !9650

Enable build job for caching Gems of GitLab EE

See merge request !9712

Remove es6 file extension from JavaScript files

See merge request !9241

Review changes

- they can have owner,
- they can be edited,
- they have description,
- you can take ownership of them