Remove categories and tags from the post type list table

There's a chance that we'll make more use of categories and tags in the future, but for the moment we don't really have any need for this in the list table display and it could cause confusion because the only thing displayed is '-'.
Setup a structure to output contact email and announcement dates in the list table.

Adds two methods to individually handle the columns.
Output contact email in the post type list table
Abstract the query that we make to get announcement dates.

I'm still coming to term with using unique keys for the date data, but this feels like it will be a faster query then trying to query meta_value all the time. I could be wrong and may come back and change this at some point, but for now it works.

We'll use this in the post type list table columns as well.
Display individual dates on which the announcement will appear

The decision to format the date in the meta key was made to help make searching easier, but it does make other things a bit weirder. That said, it works.

It may be worth some extra efforto abstract the parsing and display of the dates.
Documenting is always good.
Tag v0.4 for production deploy