'addClass' is more general than just Bootstrap forms. In particular, it is
copied into the yesod-form-bootstrap4 project and I found myself using it in my
custom forms. It would be useful to have it exported for use elsewhere.
I added 'removeClass' because I needed it while creating a custom 'readonly'
input in a form and thought it might be generally useful.
* Fixes bug where @<button>@ was being treated as a Haskell function reference
* Uses '<*' instead of @<*@, since <* is a Haskell function (though this doesn't appear to make the link work)
