Add pinned courses feature to dashboard

Replaces the welcome message on the dashboard, which was pretty useless
anyway, let's be honest. The get_pin_url and get_unpin_url methods on
Course have corresponding tests, but the views don't, since the current
testing framework doesn't really support testing these views. I'll have
to update the test base classes to account for this.

Requires a db migration! (Create the wiki_userprofile_pinned_courses
table.)

Also requires compiling the CSS.