adding referrer info to contact form with JS

The server-side solutions for adding referrer info (PRs #29 and #30)
to the contact form doesn't work because we cache the contact form for 10 minutes.

This solution uses JS to add the referrer info to the UI and as a
hidden field.
fixing Referrer in contact ticket
Merge remote branch 'origin/referrer_capture_that_doesnt_break_caching'