Offer helpful RBTools setup instructions in the repository list.

This updates the repository list to have helpful action links for
getting RBTools configured against that repository. When the admin
clicks "RBTools Setup", it will pop up a modal dialog box showing the
setup instructions for that given repository.

To make room for the new column, I've also shortened the label for the
"Show this repository" column in the repository list. Upcoming changes
will add new actions to this list.

Reviewed at https://reviews.reviewboard.org/r/6306/