Fixes #809: persist active tab through user's browsing session

1. Uses sessionStorage to display the user's last active tab panel when the page is reloaded or goes through a language swap.
1. Decided to use sessionStorage because it allowed for all types of page reloads to display active panel rather than rely on binding to clicks of language swap link(s).
1. Cleaned up repeated hash logic.
1. Removed unused slider focus event handler.