Simplify types

[psrpinto]

I would recommend reviewing comit-by-commit, there's a lot of commits but they should be easy to understand on their own.

This PR simplifies the type definitions quite a bit, and also other aspects of the frontend code, so that it's easier to then implement the Navigation type.

Thanks @ashfame for providing feedback on the types being too complicated (you were definitely right, there were a lot of things wrong), and for discussing things prior and during development of the PR.

There are no user-facing chances here, just cleaning things up in preparation for importing navigation.

Summary of changes

• Post has been renamed to Subject. It represents the piece of content being imported, e.g Blog post, product, header, footer, etc. This helps with differentiating between the "liberated post" and the "transformed post". In short, subject = liberated post.
• The type definitions have been simplified quite a bit.
• Moved things around, renamed files.