Allow generation and serving of abritrary custom pages

The administrator can define pages in the admin panel defined as type
'other', and when set to active, those pages can be served from the
/pages/<page-name-slug> URL from the base of the helpdesk.

This is mocked up only against Apache
Merge pull request #616 from greezybacon/feature/custom-pages

Allow generation and serving of abritrary custom pages

Reviewed-By: Peter Rotich <peter@osticket.com>