add a django form validation for the boku service id (bug 999075)

https://bugzilla.mozilla.org/show_bug.cgi?id=999075

Waiting on @andymckay's curling update for the parse error.

![screenshot 2014-04-23 09 40 36](https://cloud.githubusercontent.com/assets/211578/2778751/46136c80-caf5-11e3-87c1-f62632a2c662.png)