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 the directory and file name be something like "dismiss-alerts" instead? It makes it seem like a bigger scope than it actually is with just "dismiss".