Don't adjust task priority after a workboard drag unless we need to

Summary:
Fixes T8197. Currently, if you priority-sort a workboard and drag a card to the top or bottom, we change the priority even if we do not need to.

For example, if the lowest priority in a column is "Low", and you drag a "Wishlist" task underneath it, we incorrectly increase the priority of the task to "Low", when we do not actually need to touch it. This is bad/confusing.

A similar thing happens when dragging a "High" priority task to the top of a column where the highest priority is currently "Normal".

Test Plan:

• Create a column with a "Normal" task.
• Sort workboard by Priority.
• Drag a "High" task above it. After patch: task still "High".
• Drag a "Wishlist" task below it. After patch: task still "Wishlist".

Also dragged a ton of tasks into the middle of other tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8197

Differential Revision: https://secure.phabricator.com/D15240