bugfix: increase circle stroke width on hover

@abersager
add 'summary' entry to filter dropdown. fix dropdown navigation.

@abersager
fix circle highlighting, extra stroke width on filter select

@abersager
re-enable hover for disabled circles

@abersager
remove placeholder images for content engagement. remove unused code.

@abersager @gtrogers
hide content explorer in IE. use placeholder element to avoid jump on data return.

@abersager @gtrogers