Register TalkOverlay to #/talk

This does the bare minimum to give back button support for the talk overlay.
Note this does not support back button behaviour from an open talk section to
the list of talk topics. This should be done in a follow up patch.

Change-Id: I829b74f2dd9b9af8fe7cf8ab65e01d9f21166c68
Merge "Register TalkOverlay to #/talk"