Force registration

This PR forces admins to register on our central server before accessing language packs or video downloads.  

Motivation:
For FLE-internal folks, please see https://forums.learningequality.org/t/forcing-registration-to-download-language-packs-and-videos/134 for a discussion.

For others, the basic idea is: we absolutely must register as many machines as possible to do impact assessment.  We know computers have to be online in order to use video downloads or language packs.  So this minimizes steps for registration (internet access is available), while giving good carrots to users.

Potential downsides:
* Not all users may have email accounts.  If this is for real, we could have a button that says "I don't care to log in, just register so I can use".  This would essentially create a headless zone.
* Offline users who are not registered, got videos through other means, and want to delete through the update interface, would no longer be able to.  I am willing to bet this is a small segment of the population :)

I will be working with @dylanjbarth to add documentation to the registration page, to give info on "Why register?" and a button to simply opt out (would require a central server API for registering without a login).