Sticky header partial updates:
handling more than one header at a level
expandable/collapsible headers as for PBC request filters

This is a WIP.  Sending to Andy for further review
Fixed issues with sticky headers where headers outside the viewport were used in header positioning computations (also reverted some prior changes as they resulted in regressions)
Fixed an issue where headers with a height taller than the "scrollaway margin" were disappearing early
Updated sticky headers to use the original header elements as the sticky ones (using the clones as placeholders)
Merge pull request #559 from reciprocity/feature/sticky_header_enhancements

Enhanced sticky headers to be more versatile, fixed bugs with clones