Enhance deviation tooltips

[jfrankl]

Detailed Description

A few recommendations from Blake:

• "population needed to match the ideal number for this district" -> "the difference between this population and the ideal population" (this change allows the copy to work in both directions, whether the current population is lower OR higher than the ideal)
• for the deviation value, a hover could provide the percentage deviation ... I find the value useful when I'm searching for other polygons to include, but the percentage is helpful when you need to know if you've achieved a target (usually expressed as a percentage)