Notifications: Started core user notification logic

Put together an initial notification.
Started logic to query and identify watchers.

app/Activity/WatchLevels.php

@@ -0,0 +1,61 @@
+<?php
+
+namespace BookStack\Activity;
+
+class WatchLevels
+{
+    /**
+     * Default level, No specific option set
+     * Typically not a stored status
+     */
+    const DEFAULT = -1;
+
+    /**
+     * Ignore all notifications.
+     */
+    const IGNORE = 0;
+
+    /**
+     * Watch for new content.
+     */
+    const NEW = 1;
+
+    /**
+     * Watch for updates and new content
+     */
+    const UPDATES = 2;
+
+    /**
+     * Watch for comments, updates and new content.
+     */
+    const COMMENTS = 3;
+
+    /**
+     * Get all the possible values as an option_name => value array.
+     */
+    public static function all(): array
+    {
+        $options = [];
+        foreach ((new \ReflectionClass(static::class))->getConstants() as $name => $value) {
+            $options[strtolower($name)] = $value;
+        }
+
+        return $options;
+    }
+
+    public static function levelNameToValue(string $level): int
+    {
+        return static::all()[$level] ?? -1;
+    }
+
+    public static function levelValueToName(int $level): string
+    {
+        foreach (static::all() as $name => $value) {
+            if ($level === $value) {
+                return $name;
+            }
+        }
+
+        return 'default';
+    }
+}