feat(client): Add email prefil on the `/signup` page if an email is passed as a URL search parameter

fixes #1285
feat(client): Add email prefill on the `/signin` page if the email address is passed as a search parameter.
Extract duplicate logic to fetch a search parameter into `BaseView.prototype.searchParam`
Merge pull request #1312 from mozilla/issue-1258-email-prefill-signup

add email prefill to /signup and /signin pages