refactor issue indexer, add some testing and fix a bug

[lunny]

This PR is a further work of #5363. It moved all issue indexer codes from models to modules/indexer/issues, added some unit tests and fix a bug caused by #5363.

[codecov-io]

Codecov Report

Merging #6131 into master will decrease coverage by 0.01%.
The diff coverage is 61.29%.

@@            Coverage Diff             @@
##           master    #6131      +/-   ##
==========================================
- Coverage   38.85%   38.84%   -0.02%     
==========================================
  Files         352      352              
  Lines       50045    50049       +4     
==========================================
- Hits        19447    19441       -6     
- Misses      27782    27793      +11     
+ Partials     2816     2815       -1

Impacted Files	Coverage Δ
models/unit_tests.go	72.8% <ø> (+1.86%)	⬆️
models/issue.go	47.59% <0%> (-0.08%)	⬇️
routers/api/v1/repo/issue.go	23.46% <0%> (ø)	⬆️
routers/init.go	68.49% <0%> (ø)	⬆️
routers/repo/issue.go	36.42% <100%> (ø)	⬆️
modules/notification/indexer/indexer.go	56.6% <62.5%> (ø)	⬆️
modules/indexer/issues/indexer.go	62.74% <62.74%> (ø)
modules/indexer/issues/queue_disk.go	49.18% <66.66%> (-3.37%)	⬇️
models/unit.go	0% <0%> (-14.29%)	⬇️
routers/repo/view.go	44.54% <0%> (-1.15%)	⬇️
... and 2 more

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