Entry resource CRUD

www.agileplannerapp.com/boards/173808/cards/7897

As a search and browse developer
I want to be able to discover the `content_ids` of topics
so that I can tag content to those topics

includes and `Entry` model, and controller actions to be able to submit entries, and retrieve entries belonging to a particular format.

i'll work on updating README and creating a `/doc` with input output examples if this looks good.
Is it not as simple as doing https://github.com/alphagov/url-arbiter/blob/master/app/models/reservation.rb#L6-L10?  I agree that this gem looks good for when our needs are more complex, but in this case it feels like overkill.