Add email notify for new release

[a1012112796]

Add email notify for new release

example view:

[lunny]

It's only for public repository. For private repository link, we need another PRs.

[codecov-commenter]

Codecov Report

Merging #12463 into master will increase coverage by 0.02%.
The diff coverage is 76.31%.

@@            Coverage Diff             @@
##           master   #12463      +/-   ##
==========================================
+ Coverage   43.42%   43.44%   +0.02%     
==========================================
  Files         643      644       +1     
  Lines       71165    71203      +38     
==========================================
+ Hits        30904    30935      +31     
- Misses      35254    35258       +4     
- Partials     5007     5010       +3     

Impacted Files	Coverage Δ
models/release.go	54.69% <ø> (ø)
modules/notification/mail/mail.go	34.48% <57.14%> (+1.98%)	⬆️
services/mailer/mail_release.go	80.64% <80.64%> (ø)
modules/indexer/stats/queue.go	52.94% <0.00%> (-23.53%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/git/blame.go	75.43% <0.00%> (-1.76%)	⬇️
models/issue_comment.go	53.90% <0.00%> (+0.15%)	⬆️
services/pull/pull.go	42.03% <0.00%> (+0.46%)	⬆️
modules/git/repo.go	49.74% <0.00%> (+0.50%)	⬆️
... and 2 more

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