Especially for Rails, I've found that these, the bulma-rails and font-awesome-rails are the most bulletproof way to set this up and not have boring lookalike Rails/Bootstrap apps.
Note that for the Rails flash messages to be dismissable just tag them with this JavaScript:
onClick="document.getElementById('notificationMessage').classList.remove('is-active')">That just strips the 'is-active' tag from the modal and hides it.
See the giantravens/rails5template for an example of it all working together.