Move all mail related codes from models to services/mailer (#7200)

* 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

models/issue_user_test.go

@@ -50,7 +50,7 @@ func TestUpdateIssueUsersByMentions(t *testing.T) {
 	issue := AssertExistsAndLoadBean(t, &Issue{ID: 1}).(*Issue)
 
 	uids := []int64{2, 5}
-	assert.NoError(t, UpdateIssueUsersByMentions(x, issue.ID, uids))
+	assert.NoError(t, UpdateIssueUsersByMentions(DefaultDBContext(), issue.ID, uids))
 	for _, uid := range uids {
 		AssertExistsAndLoadBean(t, &IssueUser{IssueID: issue.ID, UID: uid}, "is_mentioned=1")
 	}