Workboards - add transactions for column changes

Summary:
adds ManiphestTransaction::TYPE_PROJECT_COLUMN and makes it work. Had to clean up the Javascript ever so slightly as it was sending up the string "null" when it should just omit the data in these cases. Ref T4422.

NOTE: this is overall a bit buggy - e.g. move a task Foo from column A to top of column B; refresh; task Foo is at bottom of column B and should be at top of column B - but I plan on additional diff or three to clean this up.

Test Plan: dragged tasks around columns. clicked on those tasks and saw some nice transactions.

Reviewers: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4422

Differential Revision: https://secure.phabricator.com/D8366