[mms] Permissions can now be applied per backend, rather than globally.
Use the method that exists for this
Use get() to retrieve registry configuration data
Fix returning initial_page value from get()
Cleanup initial page handling

Use Horde_Registry#getInitialPage()
Remove some unneeded logic
[mms] Add Horde_Registry_Application#getInitialPage().

Allows dynamic overriding of an application's initial page. Potential of
saving a page access/redirect when accessing an application for the first
time (i.e. login; access from topbar).
Implement Horde_Registry_Application#getInitialPage() for IMP
Implement Horde_Registry_Application#getInitialPage() for Ingo
Implement Horde_Registry_Application#getInitialPage() for Turba