Initial draft of a dismissible widget

[jonasfj]

Hmm, not sure I might have a SASS formatter that took some liberties :see_no_evil: I can look into fixing that sometime -- or we can consider adopting something like it :/

Hmm, until it's dismissed the content is a bit jumpy.. Ideally, we should make the announcement banner cover some our margin, so that the page doesn't jump whether it's visible or not. The animation was only because it always starts hidden.