Defined and use variables for .panel-heading & .panel-footer padding

Hej.

In my opinion .panel-heading's & .panel-footer's padding earn a variable in variables.less.
It came in very handy for a Bootstrap-based project of my own where I needed certain panel styles and was surprised the padding wasn't set by a var since I needed it.

So I thought I share it and you may agree.

Cheers,