No longer read the 'prop' of href for modal confirms.

The problem with the 'prop' is that when the href is '', the prop returns the
full url of the page we're on. This is probably not what is intended - normally
what you're after is just that browsers CSS-style the link as if it had a href
attr.

If you really want to programatically change the target, that's what the
data-href is for.
Add a property in Trex.opt for flagging what version of bootstrap is in use.

Trex javascript libraries can use it to render things differently depending on the bootstrap version. It defaults to 3.
Make modal forms support bootstrap 2.

Since we don't have a sane way to set the bootstrap version in one place, you have to pass it to render_modal_form for now.