* Removes the "content type" tab bar from the "Add Existing Component/Unit/Subsection" modal, and in all cases where only one type of content is shown for selection.
* Updates the section/subsection sidebar to show "Existing Library Content" + "New Subsection" / "New Unit" buttons.
* Updates the "Add New Unit/Subsection" buttons to directly launch the new container modal, instead of going via the container sidebar.
* Ensures that whenever a subsection/unit is created from within a section/subsection, that it is linked to the parent section/subsection after created.
* Adds section and subsection library pages.
* Refactors routing to support them and fix routing from collections page to other pages.
* Refactors library context to reliably set component, unit, and other container ids even when the url changes when user goes back in history rapidly.
