A warning should be displayed when estimation field is unavailable

As of now, under Configure Board > Estimate it's possible to select a field that isn't configured for Issues included in the board. It would be good if Agile checked whether the field is available in Projects included in the board and displayed an information if that isn't the case.

Example: Business Value field is only configured for Project A, but can still be selected as the estimate for a board created for Project B. No warning will show, the estimate tooltips for any of the Issues just won't be displayed.