Add Tweet filtering for retweets

What's New in v1.11.2

Commands

• /twitter retweets
  • Update followed accounts to either allow or block re-tweets
• /twitter follow
  • Add new option to not allow retweets from the start

Style

• Update tweet notifications to mention if it's a retweet
• Update /twitter unfollow to be more clear as to what to do

Backend

• Cache guilds and channels to prevent too many Discord API hits while sending tweets

Bugfixes

• Fix a bug on /twitter unfollow that caused an error message (though the command ran successfully) (thanks @GlitterButts @aliquis)
• Change config loading from the database to use raw pymongo for admin commands