Browsable API JSONField should use textarea widget

[daggaz]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

• Create a Serializer with a JSONField, and a View that uses it.
• Visit the browseable API

Note: this can't be a ModelSerializer backed by a postgres json field, as that is handled here

Expected behavior

Form input should be a <textarea>

Actual behavior

Form input is an <input type="text">