Add compare tag dropdown to releases page

[jtran]

Closes #11743.

Beside each release on the releases page, there's now a compare dropdown listing all tags. When a tag is selected, it goes to the compare page comparing the selected tag with the release's tag.

[codecov-commenter]

Codecov Report

Merging #15695 (006efdf) into master (5c5cfd6) will increase coverage by 0.01%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #15695      +/-   ##
==========================================
+ Coverage   43.89%   43.90%   +0.01%     
==========================================
  Files         678      678              
  Lines       81868    81877       +9     
==========================================
+ Hits        35933    35950      +17     
+ Misses      40086    40073      -13     
- Partials     5849     5854       +5     

Impacted Files	Coverage Δ
routers/repo/release.go	24.39% <66.66%> (+1.19%)	⬆️
modules/queue/unique_queue_disk_channel.go	53.73% <0.00%> (-1.50%)	⬇️
modules/queue/workerpool.go	51.64% <0.00%> (-1.10%)	⬇️
services/pull/pull.go	43.47% <0.00%> (-0.46%)	⬇️
services/pull/patch.go	55.93% <0.00%> (+1.69%)	⬆️
modules/process/manager.go	75.30% <0.00%> (+2.46%)	⬆️
services/pull/check.go	28.76% <0.00%> (+2.73%)	⬆️
modules/notification/mail/mail.go	41.83% <0.00%> (+3.06%)	⬆️
services/pull/temp_repo.go	29.78% <0.00%> (+3.19%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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[kdumontnu]

Works great!