R1:0a76d82a7c4d

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…