Split out the sidebar in advance of significant Latest mode only
changes.
The Less split was made by adding a new Sidebar.less file that imports
the Legacy implementation.
The Mustache split was made by file copying the existing implementation
into a legacy/ subfolder. Both the Legacy and Latest implementations
were then stripped of templates not applicable to their modes.
These changes are aligned with the splits for JavaScript.
Bug: T246419
Change-Id: Ib82769ea16bfc09efe8a088220aab55bdde0d381
