Allow enabling h export_formats feature flag per instance

[acelaya]

Enable h export_formats feature flag via instance configuration, like we did with import/export.

Testing steps

1. Disable export_formats in your local h instance: http://localhost:5000/admin/features
2. Enable export formats in your local LMS instance: http://localhost:8001/admin/instance/8/settings
   
3. Open https://hypothesis.instructure.com/courses/125/assignments/4989 and verify the formats dropdown and copy-to-clipboard button are displayed in the export pannel
   

Considerations

In the assignment linked above, copying to clipboard works, but I have seen others where it doesn't (like https://hypothesis.instructure.com/courses/125/assignments/873).

I'm still trying to figure this out, but it shouldn't block this particular PR.

EDIT: We found the reasons for the issue above: There was missing permissions that had to be set in iframes generated by LMS and via.

Additionally, the wombat.js library used by pywb in viahtml, is overwritting the permissions set by the client. We have opened an issue there: webrecorder/wombat#133

[marcospri]

The setting looks good 👍

You could enable it in one/some of the testing applications instances over https://github.com/hypothesis/devdata if you plan to keep testing this locally.

[acelaya]

You could enable it in one/some of the testing applications instances over https://github.com/hypothesis/devdata if you plan to keep testing this locally.

Thanks! I actually want this mainly to enable it in our LMS instances in QA and prod, plus have a good way to progressively enable in other instances.

In dev envs, we can just enable it globally in h.