New lookahead-tray component

You can see the component by going to a database, then clicking on
the database name. That will open up a tray where you can type to
get a list of other matching databases. Clicking <enter> or
selecting the item from the list will take you to that particular
DB page.

Closes COUCHDB-2420
Implemented feedback from @seanbarclay; fixed misc issues
Minor CSS fix.