feature(client): Smooth the `forgot password` experience from `/signin`

* Send password reset email from /signin if user has filled out a valid, known email.
* Show user the `Unknown user. Sign up.` error message if valid, unknown email.
* Send user to the `/reset_password` page and pre-fill any partial emails otw.

fixes #762
fix(test): Fix the reset password related tests that were broken by the new form prefill features in #1048. Ensure sufficient test coverage in the sign_in mocha tests.
Merge pull request #1040 from mozilla/issue-762-confirm-reset-from-signin

feature(client): Send password reset email from /signin if user has filled out a valid, known email.