This commit creates a new organization setting that determines whether
a user can invite other users to streams. Previously this was linked
to the waiting period threshold, but this was both not documented and
overly limiting.
With significant tweaks by tabbott to change the database model to not
involve two threshhold fields, edit the tests, etc.
This requires follow-up work to make the create stream policy setting
work how this code implies it should.
Fixes#12042.
David Wood