[SECURITY] Notify users about account security changes

- Currently if the password, primary mail, TOTP or security keys are
changed, no notification is made of that and makes compromising an
account a bit easier as it's essentially undetectable until the original
person tries to log in. Although other changes should be made as
well (re-authing before allowing a password change), this should go a
long way of improving the account security in Forgejo.
- Adds a mail notification for password and primary mail changes. For
the primary mail change, a mail notification is sent to the old primary
mail.
- Add a mail notification when TOTP or a security keys is removed, if no
other 2FA method is configured the mail will also contain that 2FA is
no longer needed to log into their account.
- `MakeEmailAddressPrimary` is refactored to the user service package,
as it now involves calling the mailer service.
- Unit tests added.
- Integration tests added.

models/user/user_test.go

@@ -625,6 +625,11 @@ func TestEmailTo(t *testing.T) {
 			assert.EqualValues(t, testCase.result, testUser.EmailTo())
 		})
 	}
+
+	t.Run("Override user's email", func(t *testing.T) {
+		testUser := &user_model.User{FullName: "Christine Jorgensen", Email: "christine@test.com"}
+		assert.EqualValues(t, `"Christine Jorgensen" <christine@example.org>`, testUser.EmailTo("christine@example.org"))
+	})
 }
 
 func TestDisabledUserFeatures(t *testing.T) {