Added dismissible alert plugin

This is the cleaned up version of https://github.com/wet-boew/wet-boew/pull/5932.
Please note the build will fail because of this issue: https://github.com/wet-boew/wet-boew/issues/5927
Should we use an actual h3 in the summary now rather than making it look like an h3? Heading elements are now permitted in the summary element and it could run afoul of WCAG to make something look like a heading when it actually isn't ([F2: Failure of Success Criterion 1.3.1 due to using changes in text presentation to convey information without using the appropriate markup or text](http://www.w3.org/TR/2014/NOTE-WCAG20-TECHS-20140408/F2)).

Might need to keep class="h3" on the summary element to increase the pointer size if you do go the header route. If we do use a heading then it may be a good idea to wrap the details element with a section element.