many redirect implementations

Probably should have split this commit up, it's huge:

- accept the state of "redirect to a deletion", where redirect_id is
  Some but rev_id is None. call this a "redirect"; previously this was
  an invalid state.
- GET for a deleted entity returns a 200 and a stub entity, not a 404
- to PUT a redirect, or to "revert" an entity to point at a specific
  pre-existing revision, PUT a stub entity. things are getting messy
  here... to detect this state, ensure the 'state' field is blank/none
  (this is for API usage ergonomics, where results from a GET are often
  re-used in a PUT or even POST)
- rustfmt
- maybe even more small tweaks along the way? mystery meat!

Tests are in python, not rust (and a future commit)

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