R1:f84130f9cd74

Support enabling a formal points field in Maniphest

Summary:
Ref T4427.

- New config option for labels, enabling, etc., but no UI/niceness yet.
- When enabled, add a field.
- Allow nonnegative values, including fractional values.
- EditEngine is nice and Conduit / actions basically just work with a tiny bit of extra support code.

Test Plan:
- Edited points via "Edit".
- Edited points via Conduit.
- Edited points via stacked actions.
- Tried to set "zebra" points.
- Tried to set -1…