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
Yeah, sorry, I kind of basically just pasted my code into the JS sample of another plugin because I didn't quite understand how the architecture worked. Will fix.