Implement intelligent deletion button behaviour

[barnabywalters]

Essentially, make the behaviour of the delete-column button reflect the amount of work it would take to re-create the column.

Rule of thumb: if it takes less than 10 seconds to re-create the column, delete on first click. Otherwise, bring up some sort of modal quantifying exactly what the user is deleting.

• deleting a newly created, unconfigured column of any type should take place on one click
• deleting a search column with a single term, or a subscription column with a single subscription, takes place on one click
• deleting an advanced search, filled list column, list column with items, subscription columns with subscriptions, etc. should show an inline modal quantifying exactly what will be deleted, and require confirmation