Fix for doc nav

Closes COUCHDB-2500
Fix for lookahead tray redirect problem

This fixes the following problem. To reproduce:
1. On the All Docs page of a database, edit a document.
2. Click Back.
3. Use the lookahead tray to select a new database.

Result: the page was missing content. What was going on was that the
route object ended up listening to the event multiple times, causing
the error. This now explicitly cleans up and re-assigns the listeners
for this scenario.

Closes COUCHDB-2503