Tabs: Eliminates jumpiness by eliminating hash changes on tab changes

Note that this still supports changing tabs through the URL hash (both on page load and when clicking a non-tab link). It no longer needs to trigger a hash change when the tab changes because I now have special handling for the enter key and space bar (since need to shift focus to the panel when hitting enter or space on a tab).

Note that this does not cover selecting a tab based on sessionStorage.

/cc @schindld @num3thod