feature(plugins): adds several reported content features

Ajaxifies the reporting form (form page still BC) and delete/archive
actions. Sorts un-archived reports at the top of results. More compact
report display, and clicking reported link opens page in an iframe
in colorbox.

Fixes #5379, #6082, #5380

This doesn't solve all problems but I think would be a step forward. Potential issue: when deleting/archiving, the list doesn't refresh (several ways to do this and couldn't decide on the best within Elgg's limited framework), so, e.g., you could delete 4 out of the widget and it would show none until you refreshed the page.