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/Notifications/MessageParts/ListMessageLine.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace BookStack\Activity\Notifications\MessageParts;
+
+use Illuminate\Contracts\Support\Htmlable;
+
+/**
+ * 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
+{
+    public function __construct(
+        protected array $list
+    ) {
+    }
+
+    public function toHtml(): string
+    {
+        $list = [];
+        foreach ($this->list as $header => $content) {
+            $list[] = '<strong>' . e($header) . '</strong> ' . e($content);
+        }
+        return implode("<br>\n", $list);
+    }
+}