Ask the wiki repo, not Gollum, if it's empty

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.
Fix group user removal from admin area

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add tests for remove group member feature in admin area

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Prevent redeclaration of LDAP strategy
Default the LDAP server label to LDAP
Merge branch 'fixes-from-master' into '7-4-stable'

Fixes from master

It includes:

* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1193
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1196
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1195

See merge request !1198