fix(client): No longer automatically sign in existing users on signup.

* Show them the "Account already exists. Sign in" error message instead.

fixes #877
fix(client): Fix the "Sign in" link on `/confirm_reset_password` if the user is in the force auth flow.

* The user should be returned to /force_auth instead of /sign_in

fixes #926
chore(tests): Add tests to ensure the correct client.json is returned under a variety of circumstances.

* all uppercase language
* all lowercase language
* uppercase region
* uppercase language
* multiple regions supported, only language specified
* unsupported region returns root language
* no Accept-Language header sent
* Three part locale

issue #979
Merge pull request #987 from mozilla/issue-877-existing-user-signup

fix(client): No longer automatically sign in existing users on signup.
Merge pull request #1066 from mozilla/issue-926-sign-in-link-force-auth

fix(client): Fix the "Sign in" link on `/confirm_reset_password` if the user is in the force auth flow.
Merge pull request #1074 from mozilla/issue-979-test-locale-handling

chore(tests): Add tests to ensure the correct client.json is returned under a variety of circumstances.
Tweaking jshintrc rules slightly
Merge pull request #1078 from pdehaan/jshintrc-tweak

Tweaking jshintrc rules slightly
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`.