[offline] Cache inspector reports sequentially, not in parallel

[davea]

For extremely long lists of planned reports this was causing load issues.

[dracos]

Aha, thanks, yep, looks good, one comment on whether images need it, but yeah, probably safer (wonder how slow it'll be).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.41%. Comparing base (4610b20) to head (e8fa9ce).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5204   +/-   ##
=======================================
  Coverage   82.41%   82.41%           
=======================================
  Files         407      407           
  Lines       31795    31795           
  Branches     5059     5059           
=======================================
+ Hits        26203    26204    +1     
+ Misses       4096     4095    -1     
  Partials     1496     1496           

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[davea]

Performance for 100 reports here is about the same, though strangely using the old code the counter in the banner sat at 0/100 for a long time before suddenly whizzing through to 100, whereas the new code it ticked up quite steadily as soon as the page loaded.