Integrate with Singlesignon

Singlesignon module provides functionality that's key for shared db multisites so most og_sites admins will want to use it. singlesignon requires sharing of many more tables than og_sites. Some of these tables no doubt should be added to og_sites sharing list, but others need to be at most optional. For example, will og_sites admins want to share out the roles table? TBD.

In general, more thought is needed about how to integrate these two modules.

Possibly we should consider factoring out the table sharing into a helper module that og_sites and singlesignon could both depend on.