Add Activity app to allow users to receive web and email notifications of file changes

[paladincorners]

Is your feature request related to a problem? Please describe.

OwnCloud 10 includes an "Activity" app that allows users to track file change activities online and configure notifications for any changes. It would be nice if OpenCloud implemented a similar feature as it allows users to monitor file changes and prevent any inadvertent activity.

See here: https://marketplace.owncloud.com/apps/activity

Describe the solution you'd like

Implement email notifications for file changes, allowing users to receive emails summarizing all the files added / deleted / changed in both their personal space and any shared spaces they belong to.

Describe alternatives you've considered

The Activity view is visible on a folder level in OpenCloud, but it is not possible to configure email notifications for this. The only email notifications available are related to spaces (e.g. being added / removed from a space).

Additional context

Add any other context or screenshots about the feature request here.

[michaelstingl]

(Slightly) Related: Global activities in the web UI:

• web-app-activities bundled but missing from default apps list #2089
• fix(web): add activities to default apps list #2090