Began refactoring CALDAV REPORT flow of control.

Removed calendar_functions.c and brought some of its code over
to caldav_reports.c so that we can use the same code path for all
three types of REPORTs.

This is motivated by an idea that perhaps the user interface can
use CalDAV as its REST API instead of inventing something new.  If
that doesn't work, we needed to finish CalDAV eventually anyway.