Accept GET requests for jump nav

Summary:
The URL /jump/?jump=%s now does the same thing as the POST version,
except that if it couldn't jump to anything, it loads /jump/ with the
query filled in so you can just press enter (we don't save searches
without a CSRF token).

(hsb: Sorry for stealing your task! It hadn't been updated in two months
so I figured you were likely not actively working on it.)

Test Plan:
Loaded given URL with different queries (including various
flavors of nothing). Search worked as expected.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1036

Differential Revision: https://secure.phabricator.com/D3047