Authentication overhaul

[petschekr]

Major overhaul of how authentication works.

Key points:

• Authentication methods split into reusable classes that can be easily extended to add additional authentication providers in the future
• Which authentication methods are available is configurable at runtime by admins from the admin panel without any code changes or server restarts
• Add support for logging in via CAS systems (with a built-in Georgia Tech implementation) :tada:
• Redesigned log in / sign up page to emphasize external login services
• Add a "new account" page after first login that allows the user to change their name (default filled in by OAuth services) or email (for CAS and local only) and add other login methods
• Users can now link/merge new authentication methods to existing accounts even if those accounts don't share an email address
• Update various dependencies and type definitions

Example of all authentication methods enabled:

Note: the CAS login method only works on *.hack.gt URLs

Fixes #216, fixes #212, fixes #188, fixes #114, fixes #210

[hackgbot]

Hey y'all! A deployment of this PR can be found here: https://registration-auth-overhaul.pr.hack.gt