Make BasicAuthManager more comprehensive, providing for use whether or not
the session is authenticated.  Provide more explicit control of setting and
resuming the credentials, and incorporate what was
AccountModel.getBasicAuth() as .getAccountBasicAuth().

It now seems to be working.

(Note for later: we need to move the "offline" notification out of the
model and into some view element, probably the LoginView.)
Equip PublicShareRoomModels with a sync that respects password requirement
and temporarily pushes a basic auth for it.

Hook in respect for password requirements and a form to fetch it.

This all now seems to work for getting the metadata of the share room, but
it doesn't provide for the fact that the folder and file contents are also
password protected of the contents of the share room.

I will aim, next, for a model prototype on which ShareRooms, Files, and
Folders can all be based.