Feature/retract tweets when post retracted

Based on the work of #3980 this extends to delete twitter posts also.

I'm not exactly happy with how dispatcher is handling this right now (check for twitter, check for fb, etc.) and am completely open to ideas on cleaning that up.

Both of these (#3980 and this PR) also make the assumption only 1 account is connected. It is possible to get more than 1 connected in D*, but I think (because the UI blocks it) we're only meant to have 1 of search service connected?