Tabs in sidenav were not being synced with history. Now if tab selected,
history is updated. If page is reloaded, tabs are synced to current
history. If link is clicked that updates sidenav panel, panel is updated
accordingly.

1. SideNav now handles URL for diff scene (additional path parameter).
2. initDefaultSpecState now validates specKey from path. If invalid
falls back to 4.0.
3. DiffScene now validates specKey(s) from url. If invalid falls back to
default.
4. TagScene now changes routes in a useEffect. Also returns an empty
node if methods are not available (can happens when URL contains not valid
method name after SideBar changes URL)