Restrict badge creation to vouched Mozillians

* Copy tweaks to front page to reflect new access requirement

* Profile model code to check for vouched status via mozillians.org API

* Badge creation view wrapper to check for vouched status

* Use django-constance to manage settings for all the above

* Misc other tweaks
Cache HTTP requests to the Mozillians.org API
Merge pull request #45 from lmorchard/mozillian-restriction

Restrict badge creation to vouched Mozillians