Contact Forms: Prefix all fields with a number.

Prefixing all fields by a running number let's us keep the current simple storing format while supporting having multiple fields of the same name and type. These prefixes are removed in the admin view, as well as the success message (though only if the prefix exists to not break previous feedback in admin).

Addresses https://github.com/Automattic/jetpack/issues/145
Good catch. It shouldn't be. Will update here, and the other case where it's stripped out (in subject token replacement).