Use string constants, not integer constants, to represent task status internally

Summary:
Ref T1812. I think integer constants are going to be confusing and error prone for users to interact with. For example, because we use 0-5, adding a second "open" status like "needs verification" without disrupting the existing statuses would require users to define a status with, e.g., constant `6`, but order it between constants `0` and `1`. And if they later remove statuses, they need to avoid reusing existing constants.

Instead, use more manageable string constants like "open", "resolved", etc.

We must migrate three tables:

  - The task table itself, to update task status.
  - The transaction table, to update historic status changes.
  - The saved query table, to update saved queries which specify status sets.

Test Plan:
  - Saved a query with complicated status filters.
  - Ran migrations.
  - Looked at the query, at existing tasks, and at task transactions.
  - Forced migrations to run again to verify idempotentcy/safety.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1812

Differential Revision: https://secure.phabricator.com/D8583