config_notifications improvements

* Changed notifier test buttons to not run if required field is missing
* Required fields will now highlight input box and add an icon if field is missing when test is pushed
* Changed Test Plex Media Server to Test Plex Client as it only tests the client and not the server
* Styled config_notifications to match new config_general styling

Wording corrections
Merge pull request #47 from Supremicus/feature/config_notification_improvements

Feature/config notification improvements