Refactor out LeftSidebarView
Make Editor, LeftSidebar, and RightSidebar channels using one big extensible record.
Clean up some whitespace.
Inline some LocalChannel creation logic
Reorganize Page to delegate more.
Refactor Page to accept Channels
Add a primitive Model to Page
Make it so Page only has dependencies on LocalChannel.
Reformat some commas.
Reorganize Channel to use namespaces better.
Rename some arguments.
Change ViewModeChange back to SetViewMode, and make a pattern match exhaustive.
Give preferById a type signature.
Have Page use channels.update too
Propagate currentDocId changes.
Have OpenDocId (from a file) more or less reset the page.