Have add_label rule remove previous add_label rules labels

[scacner]

The add_label rule should be used to define a column's label. If a card is moving from one column to another and both columns have this rule, the labels of both columns would now be associated with that card. There should be a feature that looks for additional add_label rules in the Project, and if there are, remove the labels of the other rules when adding the new label.

[scacner]

It might make sense to rename this rule to add_column_label to help clarify that the label added would only be used for cards in that column.