- Creates a new user-preference called 'VectorSidebarVisible'
which stores the sidebar hidden/collapsed state for logged-in
users.
- Updates that user-preference on the client whenever the sidebar
is expanded or collapsed.
- Refactors the sidebar related javascript into a separate file.
Bug: T255727
Change-Id: Ib1ce934f3646cd8feebf0d3b15c38b5b969ec957