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
Aren't race conditions in JavaScript only possible when two events fire simultaneously and try to modify or get data from the DOM? As in, JS is not multithreaded, so how could Modernizr not have finished executing.