Account source in user detail 1052625

https://bugzilla.mozilla.org/show_bug.cgi?id=1052625

Add an anonymous API endpoints to check the account source for an email address. This will either return `{"source":"firefox-accounts"}` or `{"source":"unknown"}` and will not 404. This is used in https://github.com/mozilla/fireplace/pull/670.

Add `verified` to the account settings endpoint so the FxA migration can start a pre-verified FxA registration flow. No PR for fireplace yet.
oh, damn. I'm used to having a [separate `dict`](https://github.com/mstriemer/zamboni/blob/account-source-in-user-detail-1052625/mkt/constants/base.py#L141-L149). I totally didn't realise that. that's fine to have then. a comment would be good though.