Indicate optional fields in help text, rather than indicating required fields in the label (bug 1049011)

r? @ngokevin @spasovski

![screen shot 2014-08-19 at 10 03 01 am](https://cloud.githubusercontent.com/assets/23885/3968877/9b93499c-27ba-11e4-9d8a-cb2ff3a52038.png)

I hide it by default with CSS, then show it where appropriate:

https://github.com/mozilla/transonic/pull/60/files#diff-0873165014a23f2a79dee01c661f3e73R99