In which we factor our the simplest CRUD logic into a base class

This comes chronologically after https://github.com/freedomgames/parklab/pull/7 and contains commits from that pull request, so merging that one will shrink the changes in this one.

I added a little base class for the most common CRUD logic on resources which should make getting the simple stuff done even simpler.

@maiamcguinness @wilhow