user invites: Disable user invites when instance signup is disabled and on on-prem

Created by: philipp-spiess

Fixes #32123 (closed)

When an instance has signups disabled, there's no value in our feature so we might as well disable it?

Additionally this force-disables this feature on non .com instances for now.

Test plan

I configured the server as suggested below and verified that the variables have the correct value by adding console.log statements: