Change of Twitter API access

[mworrell]

Reading:

https://twittercommunity.com/t/announcing-new-access-tiers-for-the-twitter-api/188728

I conclude that we have no future for the code to import tweets.

If that is true (and it sure looks like it) then I propose to remove mod_twitter completely. Looking at how Twitter is developing I also see no reason to let people use Twitter as an authentication provider.

[mworrell]

People who previously used Twitter as their authentication provider can continue using the username/password that has been created during signup.

[mworrell]

Deleted mod_twitter in #3484