add discard draft method

[johndoh]

closes #10184

Add a method to discard a saved draft, cleaning up the compose data in the session.

I put the discard button at the other end of the toolbar to prevent accident click problem if its too close to save.

[alecpl]

I'm not sure I like this button there.

Maybe it should get integrated with a Cancel button (#6251), e.g. as a question after clicking Cancel.

With requests like #5937, maybe there's a need for some "Actions" button near by the Send button. Would make sense if there's at least two extra actions though. "Send and Archive" is a plugin feature, so it might leave the Discard alone.

[johndoh]

Should I remove the skin change so the function can be added to the core, then people can make UI suggestions?

I wondered if may be it could go at the bottom next to the open in new window link on big screens and in the menu on small screens: