Add alert icon, like exclamation mark in black circle, to call out and alert styles

[ElizabethPace]

At Haringey we've noticed that some people ignore blue call outs, where we put info we want to draw to their attention, and skip over them to get to the next section of text. We think people may also be ignoring yellow and red alert styling, although we want to test this.

Is it possible for callout and alert styling to do something like GOV.UK do which is add a black circle with an exclamation mark to the callout/alert? We've noticed this is ignored less. We then aren't just relying on colour to draw someone's attention to something important.

[markconroy]

I like this idea. Moving it to Localgov Base to work on it.

[markconroy]

PR for this here: #562

Here's how things will look with this PR applied:

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Thanks to Big Blue Door for sponsoring my time to work on this.

[markconroy]

@msayoung We should chat about this one. It would be good to get a consensus on it so we can work on it, or close it.