user invites on-prem: Add site or user configuration option to turn off user invitations on on-prem

Created by: philipp-spiess

Part of https://github.com/sourcegraph/sourcegraph/issues/31214

See https://sourcegraph.slack.com/archives/C01LZKLRF0C/p1645585709938119?thread_ts=1645565266.889989&cid=C01LZKLRF0C for more context