includes/messages.html should show different color alert depending on message type

See here:
https://github.com/CodeSelfStudy/codeselfstudy_flask/blob/feature-db/app/codeselfstudy/templates/includes/messages.html

[Available classes](https://bootswatch.com/readable/) are:
- alert-info (current class)
- alert-success (e.g., for successful form submission)
- alert-warning
- alert-danger (e.g., for errors)
