allow widgets to specify what hubs they can be added to
Adds a new optional property that can be set in a widget to
allow a widget to specify what hub types it can be added to.
The new property is a list of the hub types it can be added to,
for example: ['user'] to only be able to be added to a user hub.
By default, the new property is set to all hub types, i.e. ['user',
'group']
Signed-off-by: Ryan Lerch <rlerch@redhat.com>