Attach the maximum external service count to licenses and enforce it

Created by: flying-robot

This completes the scaffolding started in https://github.com/sourcegraph/sourcegraph/pull/14916 and enforces the maximum number of external services associated with the current license.

Here's a local example where I switched to enterprise mode and tried to add another external service atop the seed data, and it was rejected:

Currently only our "team" plan will restrict this value in a meaningful way, but as plans evolve over time the switch statement will expand and change.

fixes https://github.com/sourcegraph/sourcegraph/issues/14024