Use hash-history mode.

The access method used by some Pancakes users obfuscates application
URLs, which likely means that history-based routing is out.  My hope is
that said mechanism doesn't interact badly with the hash mode.

(If it does, that access method will have to go.)
Deactivate SC selection from the routes that set it.

This smells weird, but deactivate seems to be the only hook that's run
on every route transition.  I suppose this can be fixed up by making a
"permits SC selection" route subclass.
A link to the "start a new search" page.

NOTE: There's currently an error (lurking somewhere; I can't yet tell)
where the study locations are not correctly pre-populated.

This isn't a release blocker (merely an inconvenience) so I'll address
it for a future release.