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

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

fixes #877
Let's lint the app/test/ files
fix(tests): Fix the jshint errors and warnings in all of the tests.

* Remove all of the remaining `global: *` declarations
* Remove or comment out all veriables that are "defined but never used"
* Use dot notation where it is "better" (very subjective)
* Make unreachable code reachable.
A couple more tweaks from Shanes meta-PR
fix(tests): Add a missing semicolon.
Merge pull request #1072 from mozilla/molint-moproblems

Molint moproblems

Travis is green. I'm merging. Thanks for this effort @pdehaan!
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
Mistake in l10n README file
Merge pull request #1080 from webplatform/master

Mistake in l10n README file
Adding explainations on how to work with i10n
Fixing markdown mistake
chore(l10n): add email marketing strings

Fixes #1042.
Merge pull request #1089 from mozilla/issue-1042-email-marketing-strings

chore(l10n): add email marketing strings
Merge pull request #1084 from webplatform/master

Improved i10n documentation