Add language and date ranges to Paste queries

Summary:
Ref T2625. Ref T3273. This is mostly a UI foil for T3273. Right now, to find tasks without owners or without projects you search for the magic strings "upforgrabs" and "noproject". Unsurprisingly, no users have ever figured this out. I want to get rid of it. Instead, these interfaces will look like:

      Assigned: [ Type a user name... ]
                [ X ] Find unassigned tasks.
      Projects: [ Type a project name... ]
                [ X ] Find tasks with no projects.

Seems reasonable, I think?

Test Plan: Searched for "rainbow, js", "rainbow + no language", "no language", date ranges, etc.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2625, T3273

Differential Revision: https://secure.phabricator.com/D6085