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
By the way, the issue discussing headings in summary elements is at: #4965. Would be good to update that issue once a solution is in place (looks like it is a combination of adding the class to the summary element and the global summary descendent fix).