[UX] List modules that need to be enabled on the config sync page

I find it frustrating that when importing config files I have to get all the way to a failed import before I find out that a module needs to be enabled, and if there's more than one, I have to deal with them ONE AT A TIME -- which is even more frustrating.

If we were able to show people on the synchronize page a list of unclaimed config files for modules that need to be enabled that would save me a lot of frustration.

Note: after https://github.com/backdrop/backdrop-issues/issues/1811 gets in, we can list the actual names of the modules and not just the names of their settings files (and a button to enable them!). But even the settings file names here would be a huge improvement.