Add ability to list import maps

[wkillerud]

For onboarding a new application it would be great to have some way to list the import maps that are available on the server.

I imagine going to https://eik.example.com/map/ to see a list. From there I should be able to find my way to the individual map to see whether it's relevant to use in my application.

At the moment we assume app developers can find their way to some sort of shared repo in the organisation to see import maps, or perhaps they just copy/paste some existing config that may or may not be optimal.