This commit adds
 - feature specs
  - to test the ability of a user with "developer" permission
    to delete tags in repositories.
 - documentation

Error messages from custom pre-receive hooks now appear in the GitLab
UI.

This is re-enabling a feature that had been disabled in merge request
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18646

The feature had been disabled due to security concerns that information
which was not intended to be public (like stack traces) would leak into
public view.

PreReceiveErrors (from pre-receive, post-receive and update custom
hooks) are now filtered for messages that have been prefixed in a
particular way.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48132

Replaces all the explicit include metadata syntax in the specs (tag:
true) into the implicit one (:tag).
Added a cop to prevent future errors and handle autocorrection.

Closes gitaly#562

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

Change single `login_via` use to `gitlab_sign_in_via`

This reverts commit 3e991230.

# Conflicts:
#	app/models/project.rb

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