New primary and secondary link behavior for Echo

See https://www.mediawiki.org/wiki/Echo/Feature_requirements#Flyout
and https://www.mediawiki.org/wiki/Echo/Feature_requirements#HTML_single_email_notifications

Also includes some related style changes per Vibha

Also removing feedback link from overlay since it's no longer needed

Change-Id: I94aed697e87126ed8ad6d1d416c99bafaa53ac53
Merge "New primary and secondary link behavior for Echo"