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.
To make type_id a keyword argument the signature needs to have a default like ```type_id=None``` in the signature. Once you do that you'll have trouble with *args afterwards because positional arguments can't come after keyword argument. It needs to be a keyword argument because ?P is used in urls.py. Maybe it's better to adjust urls.py to use a positional argument using parentheses instead. It's up to you but something needs to be adjusted.