Centralise skipColumnHeader to one function (fixes notices, cleans code)

[eileenmcnaughton]

Overview

Centralise skipColumnHeader to one function (fixes notices, cleans code)

Before

Notices & duplicated code on preview screen for import

After

Preview screen still handles the column headers (error is due to csv missing date data)

Technical Details

This adds a parent class that allows the preview screen (and soon other screens in the flow) to access values submitted on the first screen in the flow - much of the confusion in this code is because of the way in which values are passed from form to form - however the quick form layer is already storing and caching this so we don't need another layer to pass the values on top

Comments

[civibot]

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[colemanw]

Ok, this code cleanup makes sense and @eileenmcnaughton's going to be doing more followups in this area so it should get run multiple times this release cycle.