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
Anyway the original point still stands, if it looks like a heading then it should use heading markup. If it's not using heading markup, then it shouldn't look like a heading. WCAG 2.0 doesn't give options around this so neither should WET.