Goals: Add initial infrastructure

This sets the basics for working with goals.

Now it is possible to apply goals to and remove goals from all items that
support being tagged using the same form and links that can be used for tags.

Fixes bug 3105
Goals: Add management command for adding project goals from a file

Fixes bug 3092
Goals: Add goals tab in TP overview

Fixes bug 3073
Goals: Cache goal stats for TPs

Fixes bug 3113
WORKING: Goal overview page

Fixes bug 3094