feature(client): Error and tooltip fixes!

* Hide errors on form change if form is valid. Help with issue #765 where "Incorrect password" error is shown even after the user updates the password.
* Prevent form re-submission while form is disabled.
* Allow tooltips to be displayed when clicking "next" button even if form is disabled.

fixes #765
fixes #858
refactor(l10n): split string to make l10n easier
chore(l10): update string templates
Merge pull request #886 from mozilla/update-string-templates

Update string templates

I give this PR an r+. Thanks @zaach!
Merge pull request #839 from mozilla/issue-765-remove-error

feature(client): Error and tooltip fixes!
feature(styles): more animations
Merge pull request #859 from mozilla/issue-734

feature(styles): more animations
feature(client): Add a `/cookies_disabled` page.

* Add a cookies_disabled template, view, and test. Provides a link to SUMO to get help as well as a "try again" button.
* Split off most of the code from main.js into lib/start-app.js so that it can be tested.
* Split off config loading into its own modules, lib/config-loader.js so that it can be shared.

fixes #738