Added filtering for PBC requests
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
tree filter controller (missing from previous commits)
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
Fixed error on blanking date field
WIP - attempting to make clone filters look like originals in all cases
Merge branch 'develop' into feature/pbc-filters

Conflicts:
	src/ggrc/assets/mustache/audits/tree.mustache
Fixed a merge typo
Updated sticky headers to use the original header elements as the sticky ones (using the clones as placeholders)
Merge branch 'develop' into feature/pbc-filters

Conflicts:
	src/ggrc/assets/assets.yaml
	src/ggrc/assets/javascripts/sticky_header.js
	src/ggrc/assets/mustache/requests/tree.mustache
Fixed assignee.name PBC request filtering (also added a fallback to email for users without a name)
Merge pull request #637 from reciprocity/feature/pbc-filters

Added filters for PBC requests