Fix #2277: make Automatic Allocation checkbox set "task activity" mode

Clicks on the "Automatic Allocation" checkbox also force the allocation mode preference to "distribute attribute points based on task activity", hopefully addressing #2277 and maintaining close to old behavior for users accustomed to the original auto-allocation setting.

@lefnire: This feels like a hella weird solution, throwing both `ng-change` and `ng-click` on the same element, but doing an `&&` in the `set` caused the `automaticAllocation` preference to stop saving to the user. Shrug!