ENH: Add support for Button classes from Theme

## Is your feature request related to a problem?
**Please describe.**
As noted here: https://github.com/DNNCommunity/Dnn.CommunityForums/issues/831
It would be good is one could set the CSS classes rendered for buttons used by the module.
This would allow a Theme to use Bootstrap or some other frameworks CSS classes.

## Describe the solution you'd like
Right now the tokens are like:
[ADDREPLY] or [SPLITBUTTONS]

I suggest that we add support for a new token [BUTTON] with some options.

A button sould ideally also render a class with it's role.

```
dcf-button-reply
dcf-button-split
etc.
```

[BUTTON.Reply] would add the current DNN classes