fix(server): Add maxAge cache control for static assets

- add configuration option for max age of static assets
- update static middleware to use maxAge value

ref #656
fix(server): add cache-control header for /config
chore(config): disable cache locally
removing extended markdown timestamps in legal docs
Helps if we call replace:tos_pp somewhere
updating shrinkwrap
Merge pull request #930 from pdehaan/issue-928

Removing extended markdown timestamps in legal docs
adding stubs for 500 or 503 pages
feature(templates): generate static 500 error pages with a grunt task
fix(l10n): ensure the default locale is listed as supported
Merge pull request #934 from mozilla/issue-933-en-us-supported-locale

fix(l10n): ensure the default locale is listed as supported
Merge pull request #913 from mozilla/issue-690

Static 500, 503 error pages

This looks great @zaach! Thanks for working through it with me.

r+

@mostlygeek - does nginx need to be updated to respond with the static 500.html page if the node server for some reason stops responding?
Merge pull request #873 from mozilla/issue656-static-asset-caching

Merge in fixes for cache control
feature(client): Add a spinner!

Displayed on:
* App startup
* 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