Clean up the login, maintenance login and required-top-level-login code.

We now have two clear and separate login approaches:
  login/ajax
  login/html

Choose the one that's appropriate.  Totally simplified the maintenance
page to be separate from the theme and dead simple, and use login/html
approach there.  Totally simplified the top level login
(login_page.html.php) to just be a login page, not the rest of the
chrome on the page and use the login/ajax approach there.

Don't use access::required in albums and then catch the exception,
instead use access::can and check the return code.

Improve the text for maintenance mode.

modules/user/views/login.html.php

@@ -1,7 +1,7 @@
 <?php defined("SYSPATH") or die("No direct script access.") ?>
 <ul id="gLoginMenu">
   <? if ($user->guest): ?>
-  <li class="first"><a href="<?= url::site("login") ?>"
+  <li class="first"><a href="<?= url::site("login/ajax") ?>"
       title="<?= t("Login to Gallery") ?>"
       id="gLoginLink"><?= t("Login") ?></a></li>
   <? else: ?>