feature(client): Pre-fill password on /signin when coming from /signup if user entered an existing account.

Other items:

* Add more unit tests to signin and signup
* Whenever focusing an element, place the cursor at the end of the text.
feature(client): Prefill form fields if a user changes from `/signin` to `/signup`.

* If both the email and password are filled out, autofocus the year.
* /signup form fields are pre-filled if user clicks on any /signup link from /signin
feature(client): Prefill email address on /reset_password for users who come from /signin

issue #1047
fix(client): Prefill the user's email on /signin if the user clicks the "sign in" link from "/confirm_reset_password"

fixes #1038
fix(client): Fix email field focus in Chrome. No more exceptions.

* Chrome has a bug where setting the selectionStart or selectionEnd of an input[type=email] element throws an exception. See https://code.google.com/p/chromium/issues/detail?id=369499
cleanup(client): Rename focus* variable names to shouldFocus* in sign_up.js for clarity.
cleanup(client): Standardize on `AuthErrors`. Convert all `authErrors`.
If the user enters an unknown email address in `/reset_password`, give them the option to go to `/signup` and pre-fill the email address.

fixes #1046
Merge pull request #1048 from mozilla/issue-1047-prefill-everywhere

form prefill everywhere