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.
@bmbouter, I was thinking that maybe that could be an endpoint for the rpm plugin to do something? I agree, that in our code we don't have anything there, so even if it is used, we cannot do a reverse look up. I mostly wanted to draw your attention to this.