Move many task status hardcodes into ManiphestTaskStatus

Summary:
Ref T1812. This cleans up most of the easy hard-coded references to specific statuses:

• The "fixes" language moves into ManiphestTaskStatus.
• Add a method to list open statuses.
• Add a method to test if a status is open.
• Add a method to get default status for new tasks.

Test Plan: Browsed around, lint, grep, created, filtered and updated tasks.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8264