Fix missing redirect to new API browser

[thll]

Adds a redirect from the old /api/api-browser route to /api-browser so that bookmarks to the old page still lead to a working API browser.

The old API browser was served from underneath the /api path and was not part of the frontend react app. The new API browser is integrated in the react app, so the backend has to serve a redirect to the frontend route.

Requires a backport to 7.1

[thll]

@dennisoelkers, there's one thing worth mentioning: if users bookmarked a link to a specific node's API browser, they will now be redirected to the global browser, because we don't have a node-specific one anymore. If they used the API browser to explicitly target API operations on that bookmarked node, this might be unexpected for them.

The alternative to the current "dumb" redirect would be to present users with a minimal landing page, stating that this functionality is gone and directing them to the new browser. I didn't implement this for now and rather wanted to discuss that. The API browser we are redirecting to looks completely different to the old one, so I figured that should be a strong enough signal for users that they are on a different page now. However, if you have concerns, we could instead implement the landing page.