Extend mail actions with common actions for regular documents

As per [specification](https://my.teamraum.com/workspaces/onegov-gever-innovation-session/improvement-proposals/ogip6-umsetzung-offentlichkeitsgesetz/view) mails should now be treated like regular documents, and should therefore have the same common actions available on them. Specifically those are:

- **Properties** ("Eigenschaften Anzeigen")
- **Edit Metadata** ("Metadaten bearbeiten")

This PR implements this.