enterprise: Enforce user and external service limits
Created by: ryanslade
We now properly enforces the 10-user limit (including when users sign in via SSO rather than being explicitly created by the site admin) and X-external-services limit for non-enterprise instances.
We were setting hook functions to check enforced limits in an Init
function which set the hooks on our global database struct and in the
move away from using globals some instances were being used without the
trigger functions.
This is a minimal changes that defines the trigger functions globally instead.
Old init function: https://github.com/sourcegraph/sourcegraph/blob/9e35730ee86edfa25c50623db13fe83ea57ceb15/enterprise/cmd/frontend/internal/licensing/init/init.go#L28-L38
An example where we create a store without the hooks: https://github.com/sourcegraph/sourcegraph/blob/243e44d5079e4d9970955f5828a0ac0bf265f5b4/internal/database/external_accounts.go#L266