Update links for Facebook and Twitter logins.
Stash work so far.

Provide initial Facebook OAuth2 support. Note: **this does not work.**  It
turns out the OAuth2 standard allows for some leeway in implementation,
meaning that Facebook's implementation is incompatible with the existing
Google implementation.  Specifically, Facebook's token URL requires a code
returned from the initial authentication request, along with the client
secret, client id, and a fresh redirect URL.  Google's implementation
requires *none* of these.

A refactoring will be required to share what code *is* common between the
two implementations, vs. that which is implementation-specific.