Improve the access denied (403) page

This is an improved version of the current access denied (403) page based on the built-in not found (404) page.

Shows a link to the CMS backend if the user is logged in and doesn't have permissions for the backend page it's trying to access, but does have permissions for the CMS backend.

Please note that the following languages are machine translated: ja, pt-br, ru and sv.
Merge pull request #314 from Naxiz/improved-403

Improve the access denied (403) page