Only affects zulipchat, by being based on the BILLING_ENABLED setting.
The restricted backends in this commit are
- AzureAD - restricted to Standard plan
- SAML - restricted to Plus plan, although it was already practically
restricted due to requiring server-side configuration to be done by us
This restriction is placed upon **enabling** a backend - so
organizations that already have a backend enabled, will continue to be
able to use it. This allows us to make exceptions and enable a backend
for an org manually via the shell, and to grandfather organizations into
keeping the backend they have been relying on.
da9e4e6e54