Use a unique random key to identify queries, not a sequential ID

Summary:
We save search information and then redirect to a "/search/<query_id>/" URI in
order to make search URIs short and bookmarkable, and save query data for
analysis/improvement of search results.

Currently, there's a vague object enumeration security issue with using
sequential IDs to identify searches, where non-admins can see searches other
users have performed. This isn't really too concerning but we lose nothing by
using random keys from a large ID space instead.

• Drop 'authorPHID', which was unused anyway, so searches can not be

personally identified, even by admins.

• Identify searches by random hash keys, not sequential IDs.
• Map old queries' keys to their IDs so we don't break any existing bookmarked

URIs.

Test Plan: Ran several searches, got redirected to URIs with random hashes from
a large ID space rather than sequential integers.

Reviewers: arice, btrahan

Reviewed By: arice

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1587