Add "add email" button to manage page

[vthunder]

I've heard multiple times from confused users that they'd like to be able to add an email from the manage page on browserid.org.

I know it's not the common case for most users, but for the ones that go exploring (and currently that's a lot, since a lot of web devs are trying it out and exploring the tech) it's sometimes a bit confusing that there's no way to add an email from there.

[shane-tomlinson]

Closing as duplicate of #421.

[ghost]

Verified as duplicate of #421.