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
That's not the only reason they have a heading. Screen readers and users of the HTML5 outline can quickly find the alerts when there is a heading and not when there is a summary element. If it is important to single out alerts so much visually then the same should be done with the outline. In the style guide we are actively encouraging the use of section/headings for alerts.