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
Goals: Enable translate links in goals tab

Fixes bug 3098
Goals: Provide drill down on goals tab

Fixes bug 3107
Goals: Provide a form for editing the goal on each drill down view

Fixes bug 3094
Goals: Show most important incomplete goal on summary

Fixes bug 3117
Goals: Show goal on the breadcrumbs when drilling down

Show goal even in file overview if the user reached it using goal drill down.

Fixes bug 3116
Docs: Provide docs for goals