Improve the UX of the 'Include archived content' exposed Views filter.

I'm currently in the process of adding an "Include archived content" exposed Views filter on several of the Open Atrium widgets. I've already done the "Task list" and "User tasks" from Work Tracker.

Out of the box, if you put a "Flagged" exposed filter (like "Include archived content") on a View, it is a select box with options: "True", "False" and "-All-". This isn't very user friendly - I even have to think about it for a moment to know which option I want!

This PR will change those options to be "Yes", "No" and "Only archived content" which is much easier to understand.

Soon, I'll be submitting some other PRs to add a "Include archived content" exposed Views filter to other widgets in Open Atrium.