Tabbed interface: URL hash used to select default tab

The URL hash has been given precedence when determining the default tab
to display.  Order of determining default tab is now:

1. URL hash that matches tab panel ID.
2. Last tab panel opened during user's session.
3. Tab with the .default CSS class.
4. First tab panel.
Merge pull request #2466 from patheard/default-tab

Tabbed interface: URL hash used to select default tab