Put configuration in database + user-interface

Finally, a solution for #24.

One reason for using `RailsSettings::CachedSettings` is that it caches by default, which is very suitable for these settings. Since the caching strategy is on-disk [by default](http://guides.rubyonrails.org/caching_with_rails.html#activesupport-cache-filestore), this should work between processes too. Another reason is that we are using key-value pairs already.

There are some helpers for easily creating form elements, which may evolve into a DSL at some point, but for now I'm happy to have this.