Provide notification for users when some has changed in the app

Right now, a user has to go back into the app to see if any relevant updates have happened, e.g. someone signed up for an event.

We should probably send emails, or at least some kind of notification to the users so they don't have to constantly check the app for updates.

No files or tests are named. Start by tracing where event sign-ups and other relevant updates are recorded, then identify any existing user-contact or notification mechanisms. Done requires an agreed notification scope and a tested way to notify users without requiring them to reopen the app.

Written by the indexing model from the issue text.