Break up `Content` type into mutliple types

[cqnykamp]

NOTE: Only address this once we have sufficient test coverage we can be sure there are not regressions! This is a major refactor.

The type Content does too much. We should split it up based on what aspects of the resource a particular page cares about. For example, we could be treating it as any one of these:

• a node in a folder hierarchy
• a shareable item
• an assignment
• an object that has certain classifications
• ...and probably more

Think about: What are the ways we view a piece of content for each page? Split up the type based on that.