Fixes T10912. When you drag tasks within a milestone, we currently apply an overbroad, API-focused rule and add the parent board's project. This logic was added fairly recently, as part of T6027, to improve the behavior of API-originated moves.

Later on, this causes the task to toggle in and out of the parent project on every alternate drag.

This logic is also partially duplicated in the MoveController.

• Add test coverage for this interaction.
• Fix the logic so it accounts for subproject / milestone columns correctly.
• Put all of the logic into the TransactionEditor, so the API gets the exact same rules.

• Added a failing test and made it pass.
• Dragged tasks around within a milestone column:
  • Before patch: they got bogus project swaps on every other move.
  • After patch: projects didn't change (correct).
• Dragged tasks around between normal and milestone columns.
  • Before patch: worked properly.
  • After patch: still works properly.

Here's what the bad changes look like, the task is swapping projects with every other move:

The "every other" is because the logic was trying to do this:

• Add both the parent and milestone project.
• Whichever one exists already gets dropped from the change list because it would have no effect.
• The other one then applies.
• In applying, it forces removal of the first one.
• Then this process repeats in the other direction the next time through.