I should be able to invite any Querki User into a Space, given their handle

Summary: Currently, you can only invite someone by handle if you are already in Spaces with them. I think this was for UI reasons: we aren't set up to be able to prompt for unknown handles, and probably shouldn't be. But if you know the exact handle, you ought to be able to invite anyway.

We should probably just redesign the Sharing page a little. Instead of having distinct fields for email addresses vs handles/names, just have one input field that can accept any of the above. Check all inputs and prompt the same way we currently do for the handle/name field. Decide whether this is email, handle or name heuristically, based on parsing the input.

Note that this requires some significant rework of the pathways, because PersonModule.inviteMembers() expects OIDs of collaborators, not random handles -- indeed, we're only passing OIDs down, in SecurityFunctionsImpl.invite(). That said, adding a list of Handles and looking those up in inviteMembers() doesn't look like rocket science: we already do something similar for email addresses.

There is some risk of ambiguity at the UI level, if someone's handle is the same as someone else's display name. But I think we already have that bug, and it's likely to be rare.