Allow auto populate email field

[josephbona]

Allows the form's email field to be set with URL param

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[josephbona]

I signed it!

[googlebot]

CLAs look good, thanks!

[KrauseFx]

Hey @josephbona, thanks for your PR, the code looks good. Can you tell us a bit more about the use-case though, and add a line of comment into the code base on what it's used for?

[josephbona]

@KrauseFx For Barstool we had a screener form to sign up for our test flight beta onebite.app. We collected their email address there then redirected to the fastlane page and passed the email address as a url param.

I could also see this being used if you are sending out a testflight invite to an email list.