Add review pending filter on pull request overview

[jpraet]

Related to #13682

This adds a filter on the pull request overview pages that allows you to find the pull requests for which you have pending review comments. There were some concerns raised in the ticket about not overloading the UI with too many filtering options though.

From my point of view, this filter would be quite useful so you don't lose track of unsubmitted review comments.
But I think the "Review Requested" filter in #13701 is more critical (also exists on GitHub).

[codecov-io]

Codecov Report

Merging #13702 (216c227) into master (e7b47c5) will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master   #13702      +/-   ##
==========================================
- Coverage   42.19%   42.17%   -0.02%     
==========================================
  Files         697      697              
  Lines       76825    76865      +40     
==========================================
+ Hits        32418    32421       +3     
- Misses      39059    39092      +33     
- Partials     5348     5352       +4     

Impacted Files	Coverage Δ
routers/user/home.go	60.11% <0.00%> (-0.49%)	⬇️
models/issue.go	55.84% <16.12%> (-1.07%)	⬇️
routers/repo/issue.go	38.06% <93.10%> (+0.06%)	⬆️
modules/charset/charset.go	68.53% <0.00%> (-4.50%)	⬇️
models/unit.go	46.57% <0.00%> (-2.74%)	⬇️
modules/process/manager.go	72.50% <0.00%> (-2.50%)	⬇️
services/pull/pull.go	40.68% <0.00%> (-0.50%)	⬇️
services/pull/check.go	49.63% <0.00%> (+0.72%)	⬆️
modules/log/event.go	59.90% <0.00%> (+0.94%)	⬆️
modules/log/file.go	75.20% <0.00%> (+1.60%)	⬆️

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