[refactor] mailer service (#15072)

[refactor] mailer service (#15072)

* Unexport SendUserMail

* Instead of "[]*models.User" or "[]string" lists infent "[]*MailRecipient" for mailer

* adopt

* code format

* TODOs for "i18n"

* clean

* no fallback for lang -> just use english

* lint

* exec testComposeIssueCommentMessage per lang and use only emails

* rm MailRecipient

* Dont reload from users from db if you alredy have in ram

* nits

* minimize diff

Signed-off-by: 6543 <6543@obermui.de>
* localize subjects

* linter ...

* Tr extend

* start tmpl edit ...

* Apply suggestions from code review

* use translation.Locale

* improve mailIssueCommentBatch

Signed-off-by: Andrew Thornton <art27@cantab.net>
* add i18n to datas

Signed-off-by: Andrew Thornton <art27@cantab.net>
* a comment

Co-authored-by: Andrew Thornton <art27@cantab.net>