Admin and Localization features.

Admin features:
* Superusers see lessons that need content added to them, with a link to the correct admin page to create the content. Incomplete lessons are hidden from regular users.
* More UI components that integrate with the admin interface. So you can work on specific pieces of data you see in the UI more easily.

Localization features:
- Lots of additional RTL support (pretty much everything should be RTL friendly now).
- Marked more translation strings, compiled locale files for translators.
- Tasks pull sentence/phrase/vocab/etc. data from the user's set language.
- Basically, internationalization is complete for all existing features and UI components.

Misc:
- Utility function for displaying modals as notifications.
- Updates to the database -- run `fab load_data` to load in the data.