Add anonymous user ID to HubSpot forms

Created by: ebrodymoore

For RFC 185, we need to start capturing Sourcegraph cloud anonymous user ID as a hidden field in all our HubSpot forms (demo, contact).

I've added the anonymous_user_id as a hidden field to the contact and demo forms. This just needs to be filled with the anonymous user ID we embed in localStorage. This HubSpot link is similar to what might need to be done. I struggle to add a lot more knowledge behind this so @dadlerj is probably the best resource to answer more technical questions about this.