Update twitter sync code to use requests_oauthlib

Update twitter sync code to use requests_oauthlib

This simplifies the code a lot, and also adds the functionality to not
get stuck in a cron-loop when somebody specifies a non-public twitter
account as their twitter account. Instead when that happens just remove
the twitter account from the feed (storing it in a new field,
oldtwitteraccount, just to be on the safe side in case twitter does evil
things to us at some point)

Code mostly stolen from the pgeu website code.