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.

• Added test coverage.
• Visited /diffusion/1/, was redirected.
• Visited /diffusion/R1:abcdef, was redirected.
• Browsed Diffusion normally.