[Street Manager] Only activate asset layer when starting report

[davea]

Previously all map pans on /around were hitting the Street Manager API unnecessarily.

WIP - /report/new is broken at the very least, probably others.

[skip changelog]

For https://github.com/mysociety/societyworks/issues/4728

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.44%. Comparing base (1e500b4) to head (816de34).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5329   +/-   ##
=======================================
  Coverage   82.44%   82.44%           
=======================================
  Files         416      416           
  Lines       32912    32920    +8     
  Branches     5281     5281           
=======================================
+ Hits        27133    27141    +8     
  Misses       4219     4219           
  Partials     1560     1560           

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[dracos]

@davea I've added empty asset_category so relevant() doesn't error at the start and never passes (so tests should be okay now), moved your code into a roadworks function, added setting always_visible in ther eso it (should?) stay on, and added an onload check if the page is new, to activate it then. Let me know if it works for you.

[davea]

Lovely, thanks. Looks good here.