* move all mail related codes from models to modules/mailer
* fix lint
* use DBContext instead Engine
* use WithContext not WithEngine
* Use DBContext instead of Engine
* don't use defer when sess.Close()
* move DBContext to context.go and add some methods
* move mailer from modules/ to services
* fix lint
* fix tests
* fix fmt
* add gitea copyright
* fix tests
* don't expose db functions
* make code clear
* add DefaultDBContext
* fix build
* fix bug
* Include thread related headers in issue/coment mail
Make it so mail programs will group comments from an issue into the same
thread by setting Message-ID on initial issue and then using In-Reply-To
and References headers to reference that later on.
* Add tests
* more tests
* fix typo
