Fixes to improve type handling for plone.app.textfield in Lists

In Dexterity, if you have a schema.List of RichText fields (or Object field containing RichText), choosing MIME types doesn't work particularly well.

Products.TinyMCE's ``getContentType()`` cannot find the MIME type for more complicated forms, but we can just tell it what it is (see https://github.com/plone/Products.TinyMCE/pull/83), so do that. Without the TinyMCE pull request going in, this will do nothing.

And, somewhat more debatably, turn on ``force_wysiwyg``. This doesn't override a user's editor setting or anything scary, what it means is that the tinymce HTML will be embedded into the page, regardless of MIME type. So if I have a 'text/plain' field, selecting 'text/html' will cause TinyMCE to spring into life, instead of doing nothing until I save & re-edit.