use django.contrib.messages for actions

[bonzini]

There is currently almost no feedback for actions in the blue box on the right. We could use django.contrib.messages to show a little box in a corner, and fade it away one second after the page loads.

https://docs.djangoproject.com/en/4.0/ref/contrib/messages/

[bonzini]

Before doing this it's probably best to update to Bootstrap 5 (the toast component had some changes between 4 and 5). The necessary changes are:

• form-group was replaced by grid
• card collapse could be replaced by accordion? check if it works
• Navbars now require a container within
• Renamed .ml-* and .mr-* to .ms-* and .me-*.
• Data attributes for all JavaScript plugins are now namespaced