Site Editor: Remove/Deprecate edited post type and edited context state.

[youknowriad]

As the site editor is growing to become a multi page app, it doesn't really make sense to have an edited post type and context state in it.
That state is forcing us today to have a two synchronization mechanism between the url and the state causing some issues related to performance and some hidden bugs at times.

Ideally the routes would just pass the post type, post id to the editor component.

That said, in order to do that, we need to refactor the existing components and hooks to avoid using these selectors:

• getEditedPostType
• getEditedPostId
• getEditedContext
• getEditedPostContext

This issue just tracks this work.

[youknowriad]

This is not entirely complete. My hope was to move the selector calls to the routes directly rather than having the Editor component "resolve" the entity.

That said, this doesn't seem to be possible at the moment without losing some nice transitions animates (for instance when moving from the root of the site editor to the styles page). So I'm going to close this issue for now, Good progress has been made already.