UI: Add "New X" buttons to dashboard and group issue, MR and milestone indexes

To do

• Use searchable dropdown since dashboard/group can have a lot of projects. Use select2?

Before

After

As you can see, for milestones, groups are listed as well as we can now easily create group milestones.

Fixes #3544 (closed) and https://dev.gitlab.org/gitlab/gitlabhq/issues/2581