Allow CSS customizations of transaction buttons

As an Jira user I want to see the default transaction action highlighted differently than the other ones.

Obviously the default transaction depends on each workflow and we define it a the most likely transaction from the current state. Probably this should be the the first one but not necessarly.

Now, if the workflow designer could add a property to mark the default transaction, this could be transposed to a CSS class, or even an extended HTML attribute, allowing the customization via CSS rules.

This is the request to allow us to define attributes that will reach the generated HTML code in order to be able to CSS mark the default button.

The proposed solution it generic enough in order allow other use cases and without risking on impacting current implementation.