Content - ContentTypes ContentTypeResource and ContentUnitsCollections

I thought these three would share more code than they ended up sharing so I combined them. The next one looks more complex though, so decided to make this PR now.
I think this URL can be formed from a reverse on the [Django equivalent of this URL](https://github.com/pulp/pulp/blob/master/server/pulp/server/webservices/controllers/contents.py#L432).