Fixing wording issues for #1594
Make sure that current and default languages are *always* within the set of available languages.
Fixing up calls to select_best_available_language (flipped parameter order) for #1601
Remove "test" from basic_addition exercise, unlink subtitles info from topic page.
Revising stats schema, implementing limits on ndays
First pass to show download log summary for videos and language packs.
Filtering data based on # days before present
Better sorting, tighter display
Necessary parts to get setup and schema migrations working again.

Conflicts:
	kalite/management/commands/setup.py
	kalite/testing/testrunner.py
	kalite/updates/management/commands/update.py
Disable caching while debugging issue.
Cleanup while debugging #1620, including removing a duplicate call into our i18n middleware.
Fix for #1614 - make sure to specify a model's zone when it hasn't been signed yet.
Now show language no matter what, and reload available language packs upon setting

Conflicts:
	kalite/templates/updates/update_videos.html
Fixing two bugs: 1) dashed-languages could not be selected for default, and 2) dashed languages if selected caused video download to throw an error
Adding in accouncements app.
Adding code to incorporate announcements.
Deleted showing announcement body; played with styling.
Moving announcements app into python-packages
Adding migrations, including app only for central server
Setting site_wide default to True, since nothing shows unless it's site-wide!