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

# To do

- [x] Use searchable dropdown since dashboard/group can have a lot of projects. Use select2?

## Before

![Screen_Shot_2015-12-07_at_17.26.52](/uploads/22c6d6df10414f9e3e35d6cea3870486/Screen_Shot_2015-12-07_at_17.26.52.png)

## After

![Screen_Shot_2015-12-07_at_17.26.33](/uploads/02d082490ed6c83c66f052a5b601b5be/Screen_Shot_2015-12-07_at_17.26.33.png)

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

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

See merge request !1968