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

config/default.json

@@ -197,9 +197,9 @@
         "EnablePreviewModeBanner": true,
         "SkipServerCertificateVerification": false,
         "EmailNotificationContentsType": "full",
-        "LoginButtonColor": "",
-        "LoginButtonBorderColor": "",
-        "LoginButtonTextColor": ""
+        "LoginButtonColor": "#0000",
+        "LoginButtonBorderColor": "#2389D7",
+        "LoginButtonTextColor": "#2389D7"
     },
     "RateLimitSettings": {
         "Enable": false,