MM-10658 Change config fields to pointers (#9033)

* MM 10658 Change config fields to pointers (#8898)

* Change fields of config structs to pointers and set defaults

MM-10658 https://github.com/mattermost/mattermost-server/issues/8841

* Fix tests that go broken during switching config structs to pointers

MM-10658 https://github.com/mattermost/mattermost-server/issues/8841

* Apply changes of current master while switching config structs to pointers

MM-10658 https://github.com/mattermost/mattermost-server/issues/8841

* Fix new config pointer uses

* Fix app tests

* Fix mail test

* remove debugging statement

* fix TestUpdateConfig

* assign config consistently

* initialize AmazonS3Region in TestS3TestConnection

* initialize fields for TestEmailTest

* fix TestCheckMandatoryS3Fields

app/user_test.go

@@ -143,7 +143,7 @@ func TestUpdateUserToRestrictedDomain(t *testing.T) {
 	defer th.App.PermanentDeleteUser(user)
 
 	th.App.UpdateConfig(func(cfg *model.Config) {
-		cfg.TeamSettings.RestrictCreationToDomains = "foo.com"
+		*cfg.TeamSettings.RestrictCreationToDomains = "foo.com"
 	})
 
 	_, err := th.App.UpdateUser(user, false)