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/LinkedMailMessageLine.php

@@ -0,0 +1,26 @@
+<?php
+
+namespace BookStack\Activity\Notifications\MessageParts;
+
+use Illuminate\Contracts\Support\Htmlable;
+
+/**
+ * A line of text with linked text included, intended for use
+ * in MailMessages. The line should have a ':link' placeholder for
+ * where the link should be inserted within the line.
+ */
+class LinkedMailMessageLine implements Htmlable
+{
+    public function __construct(
+        protected string $url,
+        protected string $line,
+        protected string $linkText,
+    ) {
+    }
+
+    public function toHtml(): string
+    {
+        $link = '<a href="' . e($this->url) . '">' . e($this->linkText) . '</a>';
+        return str_replace(':link', $link, e($this->line));
+    }
+}