Fix endpoints for userlist management and move userlist management inside the securesync app

Fix for #1418

Currently, the API endpoints for userlist management are in the main app. Code refactoring pointed the client side to non-existent endpoints in securesync.

*Student management APIs are now embedded in the securesync/users app.
*Userlist management now embedded in securesync/users app.
*"Ungrouped" internationalized.

Gone through a bit of testing here, but a little more wouldn't hurt!

Should be working as intended now, both on distributed and central.