Allow Maniphest queries to be saved

Summary:
There have been a couple of requests for this since bookmarks are "out this year like woah" and "totally uncool dude".

Allow users to save named custom queries and make them the /maniphest/ default if they so desire.

A little messy. :/

Test Plan: Saved, edited, deleted custom queries. Made custom query default; made 'no default' default. Verified default behavior. Issued a modified search from a custom query.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley, 20after4

Maniphest Tasks: T923, T1034

Differential Revision: https://secure.phabricator.com/D1964