Round about queries to find only dates with valid announcements.

First query for any posts with this month's meta key
Then query for posts that are published or private from the results of the previous query.
Finally, query for distinct meta keys for the published posts so that we can display what we want.

We should remove private from the query as we don't have much use for it.
Remove post status private from the calendar query
Adjust the title on the calendar day link
Remove the previous and next month queries, but leave placeholders
Document new month and day link generation functions
Update documentation for our modified method
Avoid displaying announcement pages for dates in the future.
Have the caption suit our needs
Introduce a widget to display an announcements calendar
Add another class to the widget output