R1:397645b2731b

Export task point values as double, not int

Summary:
See <https://discourse.phabricator-community.org/t/maniphest-non-integer-point-values-in-csv-export/1443>.

We currently export the Maniphest "points" field as an integer, but allow it to accept decimal values (e.g. "6.25").

Also fix a bug where we wouldn't roll over from "..., X, Y, Z, AA, AB, ..." correctly for Excel column names if sheet had more than 26 columns.

Test Plan:
- Set a task point value to 6.25.
- Exported to text, JSON, XLS.