Open311 JSON fixes

[dracos]

Stop the JSON output using arrays where not needed (was fixed for requests back in 2017, looks like, but this was missed), and also include JSON links on the index page.

Fixes https://github.com/mysociety/fixmystreet-commercial/issues/2106

[codecov]

Codecov Report

Merging #3279 (cbe743e) into master (ac3d686) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3279      +/-   ##
==========================================
- Coverage   82.12%   82.03%   -0.10%     
==========================================
  Files         316      316              
  Lines       20971    20971              
  Branches     3163     3163              
==========================================
- Hits        17223    17204      -19     
- Misses       2717     2733      +16     
- Partials     1031     1034       +3     

Impacted Files	Coverage Δ
perllib/FixMyStreet/App/Controller/Open311.pm	72.18% <100.00%> (+8.28%)	⬆️
web/cobrands/fixmystreet/offline.js	31.75% <0.00%> (-13.28%)	⬇️
web/cobrands/buckinghamshire/assets.js	91.80% <0.00%> (-1.64%)	⬇️
perllib/Utils.pm	97.97% <0.00%> (-1.02%)	⬇️
perllib/FixMyStreet/App/Controller/Rss.pm	68.37% <0.00%> (-0.94%)	⬇️
web/cobrands/fixmystreet/fixmystreet.js	69.90% <0.00%> (-0.33%)	⬇️
perllib/FixMyStreet/DB/ResultSet/Contact.pm	100.00% <0.00%> (+7.14%)	⬆️

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[davea]

Looks good!