Tooltip for forks count on projects page

This issue came from: https://gitlab.com/gitlab-org/gitlab-ce/issues/20137#note_16594448

Imo this should already have been there ;) but as https://gitlab.com/gitlab-org/gitlab-ce/issues/20137 can get implemented, this should certainly be done for consistency purposes.

Currently it looks like:

It should look like:

cc: @jschatz1