Add track-seen tag support

This is part of an effort to allow analytics tracking of whether a
specific piece of content has been seen (scrolled to).

Tags can be added to govspeak content with the tag `{@track-seen:<content identifier for analytics>}`. 
A script will be added to static to find tracker elements (output as `<span data-track-seen="the identifier"></span>`) and send off analytics events when appropriate.