Support email prefill on account sign up page

[zaach]

The idea is that the browser could open about:accounts prefilled with e.g. a user's old-sync email. See https://bugzilla.mozilla.org/show_bug.cgi?id=1020156

[shane-tomlinson]

We have a model for this in app/scripts/views/force_auth.js using the email query parameter. We can extract that logic and re-use it.

[shane-tomlinson]

@zaach, @ryanfeeley, @johngruen - In this situation, would the user be able to edit the email address, or is the address read-only?

[gijsk]

@shane-tomlinson They'd still be able to edit it, see https://bugzilla.mozilla.org/show_bug.cgi?id=1020156#c0

[johngruen]

@shane-tomlinson ditto @gijsk

[ckarlof]

This feature may also be useful for Oauth flows where a relier already knows the email of the user they need to (re-)authenticate.

[shane-tomlinson]

@ckarlof - should we support this behavior on the signin page as well?

[shane-tomlinson]

I have added prefill to the signin page, we can remove it if needed.