Rapid URL should be a config parameter (rather than hardcoded)

[jake-low]

As a temporary workaround for facebook/Rapid#1561, I published a hotfix to pin the Rapid version at 2.3. This required changing the MR frontend code, tagging a new version, building a new docker artifact and redeploying the service.

It would have been easier if I had chosen to make the Rapid CDN URL a configurable value when I refactored the editor integration recently (something like REACT_APP_RAPID_EDITOR_CDN_URL in .env). So I'm opening this issue to remind myself to implement that at some point. I'm not sure if it's straightforward to embed values from the config into static assets like rapid-editor.html but I think it's probably possible.