Include category group in CSV export

[dracos]

Fixes #2994
Fixes https://github.com/mysociety/fixmystreet-commercial/issues/1874

First commit is shared with #3011 and #3012 to allow the group lookup to be done as part of the normal DB call.

[codecov]

Codecov Report

Merging #3016 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3016   +/-   ##
=======================================
  Coverage   83.02%   83.03%           
=======================================
  Files         246      246           
  Lines       15385    15376    -9     
  Branches     2869     2871    +2     
=======================================
- Hits        12774    12768    -6     
  Misses       1695     1695           
+ Partials      916      913    -3     

Impacted Files	Coverage Δ
perllib/FixMyStreet/Cobrand/Bexley.pm	85.71% <ø> (-0.55%)	⬇️
perllib/FixMyStreet/Cobrand/TfL.pm	85.12% <ø> (+0.12%)	⬆️
perllib/FixMyStreet/DB/ResultSet/Contact.pm	100.00% <ø> (ø)
perllib/FixMyStreet/App/Controller/Dashboard.pm	90.49% <100.00%> (+0.41%)	⬆️
perllib/FixMyStreet/App/Controller/Admin/Users.pm	86.18% <0.00%> (+0.23%)	⬆️

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