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
We're also making things more complicated for implementers. They see some alerts with heading markup and others without heading markup. They are going to get confused by the different examples and won't understand why they are different. We should pick one approach and stick with it rather than showing different approaches to the same alert without any explanation or justification for why they are different.