feature(client): Flash messages

issue #891
fix(build): Use the custom version of Modernizr in prod, not the full version.

fixes #949
lower static_max_age to 10 minutes until we have cacheproof urls for all static assets
chore(l10n): show missing strings for supported languages
Merge pull request #990 from mozilla/supported-locale-more-info

chore(l10n): show missing strings for supported languages
Merge pull request #991 from jrgm/static_max_age

lower static_max_age to 10 minutes until we have cacheproof urls for all static assets
refactor(test): Less auth-server logging for travis tests!

Only display errors.
chore(l10n): fix supported locales script to print correct missing strings
Merge pull request #984 from mozilla/issue-949-future-of-the-past

fix(build): Use the custom version of Modernizr in prod, not the full version.
Merge pull request #924 from mozilla/issue-891-flash-message

feature(client): Flash messages
Merge pull request #998 from mozilla/fix-supported-locales

chore(l10n): fix supported locales script to print correct missing strin...
Merge pull request #1004 from mozilla/less-auth-server-logging

refactor(test): Less auth-server logging for travis tests!
feature(client): Add a link to resend verification email for unverified users who change their password.

fixes #887
feature(client): Add Firefox for Android marketing on the confirmation screen.

fixes #900
feature(client): debounce resend button clicks.

* Send emails on the first and forth attempt. Shake the success message otherwise.
* After the forth attempt, hide the resend button for 5 minutes.
* Change the name of `.error-shake` to `.shake`.
* Update `.shake` to cause 8px of movement, not 100px.

issue #885
Merge pull request #975 from mozilla/issue-887-unverified-change-password

feature(client): Add a link to resend verification email for unverified users who change their password.
feature(client): Prevent users from seeing confirmation pages if they have not initiated the process.

* For /confirm, redirect to /signup
* For /confirm_reset_password, redirect to /reset_password

fixes #847
feature(client): Add a button spinner!

Displayed on:
* View render
* Form submit

* Add a new static function to Base - Base.withProgress. This shows a progress
 indicator if the action takes a long time.

fixes #371
cleanup(css): Extract inline color values to _variables.scss

* cleanup for @pdehaan's comments in #1003

fixes #1009
fix(client): Gracefully handle server errors when fetching translations.