fix: resolve backward compability in config

This just adds backward compability with the old configs.

When a literal string is found in the array, it's converted to a dict with just the url_prefix property to match the start of the URL, which matches the intended behavior in Libravatar before, and still without allowing open redirects.

While I was at it, I also renamed the test variables to make it clear what they were actually testing.

Closes #90 (closed)