Add extra column for "election" tag in test lists

[agrabeli]

To more easily flag and track election-related websites, perhaps we can add an extra column to the Citizen Lab test lists where contributors can add an "election" tag.

We could potentially include this in the existing "notes" column, but I think having a separate,dedicated column for tagging election-related sites will make it easier/cleaner (particularly in cases where the "notes" column already has other information).