Clean up dead code from old task list

Summary:
Removes a bunch of dead stuff:

• Old side nav with hard-coded filters.
• Old edit/list/delete/update interfaces for those filters.
• Old buildStandardPageResponse().
• Some other junk with no callsites.
• Reduce the number of places where the "Create Task" button is built.

Test Plan: grep; used list view, batch editor, reports.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D6985