feat(broadcast_message): update to use GlButton

- Using the .gl-close-btn-color-inherit class, we can now
add a close butotn that inherits its `x` icon color from
the parent and still has a hover, focus, active states.
- Update paddings and markup to have a cleaner look
and more breathing space for text.
- Instead of showing the word "Dismiss" on xs and below,
we now use the "x" icon an all breakpoints.