Graduate extensions: Sourcegraph web changes

Created by: vdavid

Closes https://github.com/sourcegraph/sourcegraph/issues/39046 Closes https://github.com/sourcegraph/sourcegraph/issues/39048 Closes https://github.com/sourcegraph/sourcegraph/issues/39049 Closes https://github.com/sourcegraph/sourcegraph/issues/39050

Listed in the order of commits:

1. https://github.com/sourcegraph/sourcegraph/issues/39046

See the first commit.

Extensions page with the flag off vs on:

2. https://github.com/sourcegraph/sourcegraph/issues/39048

See the second commit. Navbar with the flag off vs on:

3. https://github.com/sourcegraph/sourcegraph/issues/39049

Flag off vs on:

Needed to pass down a setting two levels, which is not so nice.

We can't remove the handlers yet as we still have the features behind flags. But for easier removal later, here are their uses:

• extensions:
  • useCodeMirror and its uses plus EditorState.create
  • getEnabledExtensionsForSubject and all its usages
  • mockExtension.ts
  • extension-registry.test.ts
• extensions.activeLoggers:
  • Has a single use here

4. https://github.com/sourcegraph/sourcegraph/issues/39050

This is not needed now. We don't want to disable the back end because we still want this functionality alive for now. We want to disable access to this endpoint on the front end if the flag is set to false, but for that we need no further changes because commit #1 in this PR (to fix https://github.com/sourcegraph/sourcegraph/issues/39046) already solves it.

Test plan

I've tested everything that came to mind. Check the screenshots above.

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.