Move the API URL to a config file

[lonnieezell]

This makes moving servers (and forgetting to change the value prior) to be more manageable since it doesn't break the UI and prevent you from logging into Membrr and changing that setting.

[brockf]

If the API URL is invalid, the "Settings" page should still be available. It shouldn't break the UI. Am I missing something?

We could store in a configuration file, regardless. Doesn't make a difference to me.