Allow commas in comma separated form choices

It is a common use-case to require a comma in a form choice, such as "Please specify your income: $1,000 - $2,000" etc. I think it would be preferable to use pipe separator since it is not an English grammatical character.

While we do have a very customized form page, we are still leaning on some underpinnings of wagtail.contrib.forms, so I think we will either need to provide our own choice field, or make a contribution to wagtail to support changing it from the hard-coded comma.