Form mapping method supports 18 fields

Our documented limit is 22 fields (the Scala limit).

http://www.playframework.com/documentation/2.2.x/ScalaForms

> Note: Case classes will only map up to 22 different fields, due to compiler limitations. If you have more than 22 fields in your form, you should break down your forms using lists or nested values.

But our Forms.mapping methods support 18 fields.

http://www.playframework.com/documentation/2.2.x/api/scala/index.html#play.api.data.Forms$

We need to either support 22 fields or change our docs to give the actual limit. Although it's a bit messy to do so, I think we should increase our limit to 22 so that it matches Scala.