Feature/retract tweets when post retracted

Closes & replaces #4135.  Based on the work of #3980 this extends to delete twitter posts also.

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?