R1:c2b8dd28d885

Support ID-based repository URIs, and canonicalize repository URIs

Summary:
Ref T4245. Make `/diffusion/123/` work, but redirect the user to `/diffusion/XYZ/` if the repository has a callsign.

(Right now, every repository has a callsign, so this always redirects.)

Also redirect `/R123:abcdef` if the repository has a callsign.

Also also, move the Pull garbage collector somewhere more sensible.

Test Plan:
- Added test coverage.
- Visited `/diffusion/1/`, was redirected.
- Visited `/diffusion/R1…