Send issue closure email with correct user - fixes #7124

[silverwind]

Lightly tested.

For anyone testing: Because of #5977, the closure must be done with a non-empty comment for it to send a mail.

[codecov-io]

Codecov Report

Merging #7164 into master will decrease coverage by <.01%.
The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master    #7164      +/-   ##
==========================================
- Coverage   41.76%   41.76%   -0.01%     
==========================================
  Files         448      448              
  Lines       61061    61061              
==========================================
- Hits        25503    25501       -2     
- Misses      32245    32247       +2     
  Partials     3313     3313

Impacted Files	Coverage Δ
modules/notification/mail/mail.go	22.64% <0%> (ø)	⬆️
models/issue_mail.go	54.76% <75%> (ø)	⬆️
models/repo_list.go	72.08% <0%> (-1.02%)	⬇️

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[silverwind]

Further testing reveals more existing bugs related to close/reopen mails (comment/open actions do work), I have a hard time triggering them consistently at all during testing. I will try to fix these as well and send another PR later with both fixes as this PR alone is kind of worthless on its own.