avoid caching the service worker for logged in users

[struan]

As the contents vary if the user is logged in and depending on their
permissions we should not be caching the service worker. This sets the
Vary header to 'Cookie' to avoid this.

[dracos]

I think this is fine, though caching always confuses me :) An alternative would be to Cache-Control: private or Cache-Control: max-age=0 (the latter apparently officially endorsed for SW: https://developers.google.com/web/updates/2019/09/fresher-sw ). But I think caching it for non-logged in people for a period of time (our reverse proxy would be an hour) is okay for our case, can always change in future. I assume it fixes your issue :)

[codecov]

Codecov Report

Merging #3201 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3201   +/-   ##
=======================================
  Coverage   83.80%   83.80%           
=======================================
  Files         249      249           
  Lines       15803    15804    +1     
  Branches     2970     2970           
=======================================
+ Hits        13243    13244    +1     
  Misses       1621     1621           
  Partials      939      939           

Impacted Files	Coverage Δ
perllib/FixMyStreet/App/Controller/Offline.pm	97.82% <100.00%> (+0.04%)	⬆️

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[struan]

Will squash this down to one commit before merging