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 set the attribute to something (e.g., <code>attr( "open", "open" )</code>) otherwise it is equivalent to a getAttribute call. Also should use vanilla JS instead of jQuery where possible (e.g., setAttribute instead of attr).