Adapt UserSession

- Move from App methods to UserSession methods
- Deprecate corresponding App methods

src/Model/User.php

@@ -830,6 +830,22 @@ class User
 		return DBA::update('user', $fields, ['uid' => $uid]);
 	}
 
+	/**
+	 * Returns if the given uid is valid and in the admin list
+	 *
+	 * @param int $uid
+	 *
+	 * @return bool
+	 * @throws Exception
+	 */
+	public static function isSiteAdmin(int $uid): bool
+	{
+		return DBA::exists('user', [
+			'uid'   => $uid,
+			'email' => self::getAdminEmailList()
+		]);
+	}
+
 	/**
 	 * Checks if a nickname is in the list of the forbidden nicknames
 	 *