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
I'm wary of specifying a precise use for the heading and section elements. We're not using changes in text presentation to convey information without the appropriate markup here. The summary element and the style of the alert clearly defines the semantic purpose here. It just happens to have a sizing class called h3, but that doesn't carry an intrinsic meaning.

Rather, alerts should be flexible enough in terms of styling to accomodate the use of any heading and other semantic structure elements. This could be achieved by enforcing a more rigid CSS which does not depend on arbitrarily defined text sizes like h3.