Add config option REACT_APP_URL_BASENAME

[gabalafou]

Description

This change should have been made with Bye tabs, hi URLs #389. That PR introduced URL routing to the app, assuming that the React app would always be mounted in the browser at the root path, "/". But the Conda Store server supports a URL prefix, which means that the UI app in such deployments, might be mounted at some path other than root, such as "/conda-store", for example loading the UI at https://example.com/conda-store instead of just https://example.com.

This pull request:

• Introduces a new configuration variable: REACT_APP_URL_BASENAME (which maps directly to React Router browser router opts.basename)

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

[trallard]

Thank you! will merge so I can test with server and UI!