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.
• Saw 6.25 represented accurately in exports.
• Exported an excel sheet with 27+ columns.
• Manually printed the first 200 column names to check that the algorithm looks correct.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19434