EE usage ping

If people start using EE more it would be nice to know this in realtime, so we can engage before the renewal comes up. We want the on-premises GitLab EE server to tell us the amount of active users.

Options (and disadvantages) are:

1. Have the server ping us (firewalls might block this)
2. Encode in the version check (might be confusing, can be turned off)
3. Use the version check loads as a proxy for usage (imprecise)

Not sure if we should allow users to turn this off.

In any case:

1. We should disclose that this is happening
2. We should not build this into CE

/cc @ChadMalchow