Summary: Sidebar extensions were unkeyed, which make them render inefficiently. I cleaned the api a bit here by making the extensions more idiomatic; they are now components rather then functions, so that they have their own render cycle, state, etc. They are memo-ed now as well, so that they don't have to re-render if the selected item doesn't change.
Reviewed By: nikoant
Differential Revision: D27685980
fbshipit-source-id: b133bc42061b3b8cf971792f5818810ecb80e3ea
