Lane Editor - Initial UI design

[leonardr]

The admin interface needs a way for an admin to control which lanes are enabled and which lanes offer custom lists as sublanes.

Implementing the list editor (see #112) is a prerequisite.

First pass wireframes: https://docs.google.com/presentation/d/1j3Wspa-QIGJm-VWE67-zgPQAgeJ3r408UgHwu3gauY8/edit#slide=id.p

The actual lane editor will be much simpler than these wireframes, closer to what is described in this document: https://github.com/NYPL-Simplified/Simplified/wiki/DatabaseLanes#user-interface

However there are complications not described in that document, mainly those surrounding the different languages in which a collection might be available.

[ricardoom]

Since this is tightly coupled to #112 this gets a Large assignment too.