Allow custom logo integration on client site

Administrators are allowed to upload one or more logos and then select from
the uploaded logos to set one for the client site. Logos can also be deleted
on settings->pages submission
Properly center dialog boxes
Freshen up default logo
Merge pull request #632 from greezybacon/feature/custom-logo

Allow custom logo integration on client site

Reviewed-By: Peter Rotich <peter@osticket.com>