Bug 887535 - Improve l10n for strings on privacy contact form

This makes the entire fields' labels localizable in https://www.mozilla.org/en-US/privacy/policies/firefox-os/ (I consider this as part of bug 887535).

Unfortunately this still doesn't solve the problem with placeholders defined in forms.py ('you@yourdomain.com' and ('Enter your comments...'), I tried putting those strings inside a _() but it won't work.