feat(client): Add support for "can_link_account" Desktop Channel message.

This message is called in the Desktop context before a "sign in" or
"sign up" to check with the Desktop browser whether another user has
previously logged in to the browser. If one has, Desktop presents
a warning to the user, giving her an opportunity to cancel the operation.
If the user cancels, we cancel the underlying "sign in" and "sign up"
operation with the auth server.
Merge pull request #726 from mozilla/issue-682

feat(client): Add support for "can_link_account" Desktop channel message.