Notifications: Aligned how user language is used

- This ensures content notifications are not translated to receiver
  language.
- This adds actual plaintext support for content notifications (Was
  previously just HTML as text view).
- Shares same base class across all mail notifications.
- Also cleaned up existing notification classes.

Future cleanup requested via #4501

app/Translation/LanguageManager.php

@@ -2,6 +2,7 @@
 
 namespace BookStack\Translation;
 
+use BookStack\Users\Models\User;
 use Illuminate\Http\Request;
 
 class LanguageManager
@@ -80,6 +81,15 @@ class LanguageManager
         return setting()->getUser($user, 'language', $default);
     }
 
+    /**
+     * Get the language for the given user.
+     */
+    public function getLanguageForUser(User $user): string
+    {
+        $default = config('app.locale');
+        return setting()->getUser($user, 'language', $default);
+    }
+
     /**
      * Check if the given BookStack language value is a right-to-left language.
      */