add close button to error flash

You're using `&times;` to indicate a close X, and the [participant selector](https://github.com/Tahi-project/tahi/blob/9dca1a8019a21365ebc3274a9dfe152955611d6a/app/assets/javascripts/templates/components/participant-selector.hbs#L3) uses a lowercase `x`. Let's standardize on one? /c @nummi