Add notification interface and refactor UI notifications

[lunny]

This pull request create a notification interface so that all notifications, i.e. ui, actions, webhooks, mails and etc. will only need to implement the interface. So that the code will be more easy. This is extract from #4001. It's the first step of serval pull requests from #4001. I think it will be easier to review.

[codecov-io]

Codecov Report

Merging #5085 into master will increase coverage by <.01%.
The diff coverage is 37.43%.

@@            Coverage Diff            @@
##           master   #5085      +/-   ##
=========================================
+ Coverage   37.39%   37.4%   +<.01%     
=========================================
  Files         306     307       +1     
  Lines       45397   45552     +155     
=========================================
+ Hits        16977   17037      +60     
- Misses      25962   26060      +98     
+ Partials     2458    2455       -3

Impacted Files	Coverage Δ
routers/api/v1/repo/pull.go	13.2% <0%> (-0.08%)	⬇️
models/issue.go	48.15% <0%> (-0.17%)	⬇️
routers/repo/pull_review.go	0% <0%> (ø)	⬆️
models/review.go	63.57% <100%> (+0.52%)	⬆️
routers/api/v1/repo/issue_comment.go	57.51% <100%> (+0.27%)	⬆️
routers/repo/pull.go	33.96% <100%> (ø)	⬆️
modules/notification/notification.go	28.08% <26.19%> (-50.49%)	⬇️
routers/repo/issue.go	32.14% <26.66%> (ø)	⬆️
modules/notification/ui/ui.go	58.92% <58.92%> (ø)
routers/api/v1/repo/issue.go	30.08% <66.66%> (+0.32%)	⬆️
... and 3 more

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[jonasfranz]

Only send notifications if status really change. In my understanding this will send a notification if isClosed = true and the issue is already closed.

[jonasfranz]

Thank you for this PR. I think this re-factoring is really needed!

[lunny]

@BetaCat0 done.