* Provide default crayon styles when creating banners
* Import crayons styles into /internal pages.
* Allow admins to use crayons banner styles when creating broadcasts.
* Preview banner styles with crayons.
* Add a "x" close button to announcements, fix styling to accommodate it
* Use constant and helper for broadcast banner styles
* Add VALID_BANNER_STYLES frozen constant to Broadcast class.
* Add banner_style helper for determining banner style class.
* Clean up preview CSS, import broadcast styles into preview
* Add close button click handler, clean up initializeBroadcast
* Add close functionality to announcement banner
Also hide announcement if the user has explicitly seen and "X"-ed out of it.
* Add system specs around rendering + dismissing broadcasts
* Add some (truly beautiful) JS documentation
* Add visible class to broadcast when previewed from internal
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