Add SUBJECT_PREFIX mailer config option

[zeripath]

Since the split Gogs has added a SUBJECT_PREFIX option to the mailer config. This PR implements this option.

Fix #6574

[codecov-io]

Codecov Report

Merging #6605 into master will decrease coverage by <.01%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #6605      +/-   ##
==========================================
- Coverage   40.55%   40.55%   -0.01%     
==========================================
  Files         406      406              
  Lines       54501    54506       +5     
==========================================
+ Hits        22102    22104       +2     
  Misses      29364    29364              
- Partials     3035     3038       +3

Impacted Files	Coverage Δ
modules/setting/mailer.go	50.7% <100%> (+0.7%)	⬆️
modules/mailer/mailer.go	22.51% <40%> (+0.05%)	⬆️
models/repo_list.go	66.84% <0%> (-1.06%)	⬇️
models/unit.go	14.28% <0%> (+14.28%)	⬆️

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