Highlight when objective estimate is reduced below current assignment

[amdw]

If you create an objective with a certain resource estimate, add assignments, and then reduce the estimate to a value less than the assignments, you bring about a situation where there are more resources assigned to an objective than it is actually estimated to require.

Right now, it's easy not to notice this, thereby leaving the objective over-assigned, which means that this surplus people-time gets wasted, and cannot be assigned to other objectives.

I can think of two main ways of improving this:

• Don't allow an objective's resource estimate to be reduced below the current assignment. This would mean that to reduce the estimate, you would first have to reduce the assignments.
• Continue to allow the resource estimate to be reduced below the current assignment, but then highlight the assignment in some way.

I prefer the second option. We could just turn the entire assignment red if it adds up to more than the resource estimate.

[amdw]

Side note: you can see in the assignment dialog when this has happened, as it shows a negative unassigned value in red and won't let you click OK while this remains the case. But right now it's not visible in the main period UI.