Move mailer to use a queue

[zeripath]

Move the mailer to use the generalised queue format.

This is a breakout from #8874

[guillep2k]

Only a nit.

BTW, I was thinking of configuring my mail to use file persistance and suddenly realized that we will have a versioning problem if we recklessly change some struct in the future. I thought I should mention it.

[zeripath]

Oh damn I forgot we need to actually change the message to use something that is serialisable.

[zeripath]

I need to add
7cb70d9

I.e.

git cherry-pick 7cb70d93c89da920467232457647009a831c6d05

[guillep2k]

Checked again, because of the recent changes. It passes test and test-sqlite on my dev env and seems to work alright.

[codecov-io]

Codecov Report

Merging #9789 into master will increase coverage by 0.03%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master    #9789      +/-   ##
==========================================
+ Coverage   42.18%   42.21%   +0.03%     
==========================================
  Files         603      603              
  Lines       78884    78885       +1     
==========================================
+ Hits        33275    33301      +26     
+ Misses      41531    41506      -25     
  Partials     4078     4078

Impacted Files	Coverage Δ
modules/notification/webhook/webhook.go	37.97% <100%> (ø)	⬆️
services/pull/check.go	54.54% <100%> (+5.89%)	⬆️
modules/notification/action/action.go	35.65% <100%> (ø)	⬆️
modules/notification/base/null.go	62.5% <100%> (ø)	⬆️
modules/notification/notification.go	73.33% <100%> (ø)	⬆️
modules/notification/mail/mail.go	24.63% <100%> (ø)	⬆️
modules/notification/ui/ui.go	74.46% <100%> (ø)	⬆️
services/pull/merge.go	50.34% <100%> (ø)	⬆️
modules/migrations/github.go	78.73% <50%> (+1.12%)	⬆️
... and 4 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 06d5cb3...866ee91. Read the comment docs.