Test legacy extension deprecation with native code host integrations

Created by: philipp-spiess

Native code host integrations can be configured in e.g. GitLab on prem and will automatically inject the browser extension code into the code host so that users don't have to load the browser extension.

We need to verify that this loads the proper feature detection code with our changes to the extension API.

To do that:

1. Set up an on prem gitlab version (or use a test instance if you find any)
2. Configure native code integrations and point it to an instance with extensions disabled (soon S2)
3. See if it works

/cc @muratsu @jjinnii @ryankscott