Update milestone strings to latest copy and mark them for l10n.
Remove resolved TODOs.
Make links in banner code absolute so they work on other sites.
Replace strings.py with template+view and remove <span> from categories.

Template + view will allow localizers to edit the strings via pontoon.
Update migration docs and add command for migrating data.

Instead of checking out certain commits and running migrations, instead
we're going to just move to the latest, make a few manual DB changes,
and pull in the old data from a copy of the old database.

No unit tests for the migration are included since the command will 
really only be used twice or so, and manual testing is far easier for
this particular case.
Fix broken progress bar display on detail page.
Move leaderboard to helper and hook it up on the homepage.
Hook up global stats on homepage and make Get Started login button work.