Activities list view improvements

[#1]

• Allow us to hide columns we don't need. (We don't use "Est. Lift in Revenue", and "Source" is always "Adobe Target" so we don't need that either.)
• Add new columns for Start and End dates.
• Allow us to add tags to activities to make it easier to organize them. For example: I would flag all my activities as desktop, mobile, or responsive. I would also flag them for the page template they're using and for the server they're on.

[#2]

@BevansDesignNTE Well, you got one of these things with the new UI. Still extremely limited though.