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.
I actually don't use the swapped values but it seemed useful I could pull it out. It's so if you know an account's source is `'firefox-accounts'` you can reverse that to `4` with `LOGIN_SOURCE_LOOKUP['firefox-accounts']`the same way `'firefox-accounts` is retrieved with `LOGIN_SOURCE_LOOKUP[4]`.