CLC-2127 User whitelist implementation

Addresses https://jira.media.berkeley.edu/jira/browse/CLC-2127 and its subtasks. 

Users can log in if they have logged in before, or if they have a canvas acct, or if they are on the whitelist. TODO also if they're first-year students (which requires data from the BF_STUDENT_VW view or LDAP). 

Whitelist administration is via RailsAdmin at localhost:3000/ccadmin and is simple as can be. If a user ID is in the user_whitelist table, he's on the whitelist and can log in. 

The whitelist and login check are behind a feature flag: Settings.features.user_whitelist must = true for any of the checks to apply. 

The "sorry you can't log in screen" has no UI design yet. I made an ugly and quite rude placeholder.