Consider renaming Content, ContentSource, and ContentPayload

[idg10]

In discussion a PR at #19 (comment) @mwadams wrote:

ContentSource is a reference to a specific instance of content at a slug (e.g. version/state/however you are choosing to model it); it basically consists of {Slug,ID}.

Content is the common envelope for all content; it has timestamps, author info, title, etc.

ContentPayload is the actual "content" in the content, with a known but custom schema.

I find this pretty confusing. There's a class called Content but that's not the class we use to represent "the actual content".

I think we should try to think of more self-explanatory names while we still can.

Perhaps ContentSource should become ContentReference.

If Content is an envelope, but is not the actual content, maybe it should be ContentEnvelope. With those two changes, ContentPayload could probably keep its current name.

[mwadams]

Content actually is the content though. You can happily have the content without a payload and it is still content (content that is just metadata). I think calling it an envelope was probably not conceptually helpful. It is a "base for all types of content, implemented via containment".

[mwadams]

We have actually performed this renaming, and opted for ContentReference, Content and ContentPayload. This will be resolved in PR #19