WIP: Fix review notifications and add them to mail templates

[guillep2k]

This PR fixes a panic produced while creating reviews, and adds review comments to the notification mail template.

[guillep2k]

@lunny I've marked the PR WIP for your consideration, although it's already functional.

It turns out that the review service was the wrong place to add the notifications, as they were already being added a few lines below where the comment is created.

What I'd suggest to follow the "services" model is to move most of routers/repo/pull_review.go to services/pull/review.go (which I removed because it was only calling models now). Perhaps that can be done in a future PR to keep this PR simple.

[codecov-io]

Codecov Report

Merging #8948 into master will increase coverage by 0.01%.
The diff coverage is 18.75%.

@@            Coverage Diff             @@
##           master    #8948      +/-   ##
==========================================
+ Coverage   41.23%   41.25%   +0.01%     
==========================================
  Files         547      546       -1     
  Lines       70794    70801       +7     
==========================================
+ Hits        29195    29207      +12     
+ Misses      37880    37870      -10     
- Partials     3719     3724       +5

Impacted Files	Coverage Δ
models/issue_comment.go	48.07% <ø> (+0.21%)	⬆️
routers/repo/pull_review.go	0% <0%> (ø)	⬆️
modules/notification/mail/mail.go	26.56% <0%> (-2.26%)	⬇️
services/mailer/mail.go	43.88% <26.08%> (-2.42%)	⬇️
modules/migrations/migrate.go	21.22% <0%> (-1.68%)	⬇️
modules/migrations/gitea.go	8.05% <0%> (-0.64%)	⬇️
models/repo.go	46.46% <0%> (-0.05%)	⬇️
models/repo_list.go	74.07% <0%> (+0.92%)	⬆️
models/error.go	33.86% <0%> (+1.18%)	⬆️
modules/task/migrate.go	28.94% <0%> (+3.94%)	⬆️
... and 1 more

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[guillep2k]

Example review mail:

[guillep2k]

Close in favor of #8954
I can add the template stuff later, since the work is already done. 😁