Remove sharingGroupUUID from FileIndexClientUI and add fileGroupUUID

[crspybits]

With the addition of file group move, the sharingGroupUUID in the FileIndexClientUI can be stale. i.e., the file group containing the file can no longer be in the sharing group.

Instead, I should really have the fileGroupUUID in the FileIndexClientUI-- since that can never change, and it can be joined with the FileGroup table to get the current sharing group if needed. I could too just not have a fileGroupUUID and join first against the FileIndex to get the file group.

This will need a server db migration, but it shouldn't be that big of a deal. The table can simply be flushed. It's a convenience for clients anyways-- to help them more accurately show download indicators.