Update FxA email link and give results for verified account info

Retrieve all accounts in the account info view but only list `'firefox-accounts'` or `'unknown'` as the source.

Update the FxA preverified email to send the user to sign up and prefill the email address.
I'm not sure I understand. We don't need to know about unknown, it's just "not firefox-accounts".

Previously we would return `'unknown'` or `'firefox-accounts'` because we would only find users that have `'firefox-accounts'` as their source. Now we will find any user since we need to know if they're verified so we need to filter anything other than `'firefox-accounts'` to `'unknown'` to leak as little information as possible.