Show dropdown of protected app's paths for easy selection.

[jjatinggoyal]

This PR introduces an option to pick paths for denying traffic from the protected app's available routes. Implements #39

Approach
After routes are loaded in the Protected app, it writes its available routes to the shared Redis instance. The Admin app reads the available routes for an application from the same shared Redis instance.

Users can still enter custom patterns by selecting None in Application Path field.

Note:

1. I didn't use stimulus for the small JS script, as I wanted to avoid adding stimulus-rails dependency.

[jjatinggoyal]

@lewispb please review

[jjatinggoyal]

@lewispb can you please take a look at this PR?

[jjatinggoyal]

@lewispb just bumping this up.