Notifications: Cleaned up mails, added debounce for updates

- Updated mail notification design to be a bit prettier, and extracted
  text to new lang file for translation.
- Added debounce logic for page update notifications.
- Fixed watch options not being filtered to current user.

app/Activity/Models/Comment.php

@@ -5,6 +5,7 @@ namespace BookStack\Activity\Models;
 use BookStack\App\Model;
 use BookStack\Users\Models\HasCreatorAndUpdater;
 use Illuminate\Database\Eloquent\Factories\HasFactory;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
 use Illuminate\Database\Eloquent\Relations\MorphTo;
 
 /**
@@ -35,7 +36,7 @@ class Comment extends Model implements Loggable
     /**
      * Get the parent comment this is in reply to (if existing).
      */
-    public function parent()
+    public function parent(): BelongsTo
     {
         return $this->belongsTo(Comment::class);
     }
@@ -50,20 +51,16 @@ class Comment extends Model implements Loggable
 
     /**
      * Get created date as a relative diff.
-     *
-     * @return mixed
      */
-    public function getCreatedAttribute()
+    public function getCreatedAttribute(): string
     {
         return $this->created_at->diffForHumans();
     }
 
     /**
      * Get updated date as a relative diff.
-     *
-     * @return mixed
      */
-    public function getUpdatedAttribute()
+    public function getUpdatedAttribute(): string
     {
         return $this->updated_at->diffForHumans();
     }