Add a mechanism to enable or disable the H integration launch view

Depends on:

* https://github.com/hypothesis/lms/issues/4197

We intend to expose an end-point which allows people to call H, but we don't want everyone to have access. We need a way of gating this access somehow.

### Questions

* Who do we expect to be setting this up? Support?
* Would an application instance option work?
* Should we include details of how to sign up in our "go away" message?

### "Privileged" users

There are some weasel words in the description:

> Through a **_secure_** API, enable _**privileged**_ LMS users to access data of their customers’ users.

* Do we need to disable this for accounts? Or did I just make that up?
* Do we need to limit this to users we've seen before associated with AI? Probably no - it can't work
* Do we need to limit this to courses / applications we've seen associated with AI? Yes - this makes sense - BUT WE CAN'T DO IT RIGHT NOW! As the grouping table is wrong
* Do we need to show errors for misaligned things? - No we can't tell the difference between wrong and not launched yet

### What about GUIDs?

* We need to check launch GUID agreement! - So you can't lie about GUID
* For two different "orgs" which share the same GUID - Do we have a problem?

### Implementation notes

* If a user makes a request to the end-point and they don't have the feature enabled they should get a 401 - Access denied
* We need to check this doesn't cause any monitoring issues (we sometimes use proxy HTTP codes values for this reason)