Added a management command for resetting GitHub auth tokens.

With the new Heartbleed vulnerability in the wild, GitHub recommends
resetting all authentication tokens, just to be sure. This change
introduces a management command that makes that somewhat easy.

This command will loop through all GitHub accounts that are linked to
Review Board, resetting their tokens and replacing them new, valid
tokens.

If using a registered GitHub OAuth app for Review Board, then the resets
won't require any credentials. However, for personal tokens (which is
the standard case), it will require a password, and possibly a
two-factor auth token.