Improve path selection UX

At the moment, it's only possible to configure paths in the Admin UI using a regex.

It'd be nice to take advantage of Rails routes to support picking the path from a list of application routes.

For an example of the sort of list of routes that would be useful:

`Rails.application.routes.routes.map {_1.path.to_regexp }.uniq`

We could call this directly in a single app install of MC-W, but for a two app install we'd need to have the Admin app read the list of potential apps from the Protected app. This would likely involve some sort of HTTP request from the Admin app to an API available on the Protected app.

The issue names no files or tests. Start by locating the Admin UI's regex-based path configuration and the Rails route list, then investigate how the Admin app could obtain routes from the Protected app in a two-app install. Done means paths can be selected from application routes in both single-app and two-app deployments.

Written by the indexing model from the issue text.