Feature request: Add method to setting threshold date from data entry form

[aubreyz]

Little more to be said than the title. If sleek uses threshold dates (which it does and should do) suggest make an easy way to enter those (in parallel with due dates which I don't use at all in fact)!

[dlaidig]

This looks like a duplicate of #234, right?

[ransome1]

There's going to be a separate threshold date picker in v2.0. In 1.3.1 due and threshold dates share one date picker.