This groups milestones by title for issue views like it has been done for
the milestone dashboard/project overview. Before milestones with the
same title would show up multiple times in the filter dropdown and one could
only filter per project and milestone. Now the milestone filter is based
on the title of the milestone, i.e. all issues marked with the same
milestone title are shown.
* Squash project users selectbox and users selectbox into one class
* Move from API autocomplete to GitLab internal one
* Smarter filter for project/group/all issues
* Use selectbox with searchbox for assignee/author/milestone/label
* Switch to ajax filter for issue author/assignee
