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/Activity/Notifications/MessageParts/ListMessageLine.php

@@ -3,12 +3,13 @@
 namespace BookStack\Activity\Notifications\MessageParts;
 
 use Illuminate\Contracts\Support\Htmlable;
+use Stringable;
 
 /**
  * A bullet point list of content, where the keys of the given list array
  * are bolded header elements, and the values follow.
  */
-class ListMessageLine implements Htmlable
+class ListMessageLine implements Htmlable, Stringable
 {
     public function __construct(
         protected array $list
@@ -23,4 +24,13 @@ class ListMessageLine implements Htmlable
         }
         return implode("<br>\n", $list);
     }
+
+    public function __toString(): string
+    {
+        $list = [];
+        foreach ($this->list as $header => $content) {
+            $list[] = $header . ' ' . $content;
+        }
+        return implode("\n", $list);
+    }
 }