create cloud function for waitlist

[rhelmer]

The marketing site currently hands waitlist requests off to github.com/mozilla-rally/rally-cloud-functions, we should create our own function to handle this in the firebase cloud function repo. It should handle attribution codes and user agent as well as the current email + country support.

[rhelmer]

The new parameters we are expecting to handle:

• attribution UTM codes ("source", "medium", "campaign", "term", "content")
• userAgent string

The userAgent string will be passed from the browser to the cloud function, the cloud function will be responsible for parsing it and sending something more sensible to Sendgrid (like OS and browser vs. the arcane raw UA strings)

[jepstein]

loaded on staging. need SRE to push to production