Stop storing transient activities (eg CRUD, undo, etc)

[snarfed]

Afaict these are probably the bulk of our datastore load, and cost (#1149). The difficulty is that we currently render them on user pages, with delivery counts, and those are useful to at least some users.

Another difficulty is that when we get a delete, we look for a create to see if we originally created the object and therefore have the ability to delete it.

We'd also need to be careful to only delete CRUD and other truly transient activities, not likes, reposts, etc.

Related: #1359, #1354

[snarfed]

I have a first pass at this done in the stash for #1354. Ugly though, not ready. Ideally I'd like to decouple this from #1354 itself, ie stop storing these activities but keep send reading from the datastore, but I don't think that's possible.