notifications: enable system notifications

[anikachurilova]

We might want to have in the future that the system will send automatic e-mails.

To be able to do that, we would need to have EntityResolver and EntityProxy, that are being used to resolve the identity, to handle System user.

For example, CommunityInclusionAcceptNotificationBuilder is using these entity resolvers. For the System user, the ServiceResultResolver for users will picked up. As a result, resolver will try to read user with id "system" from UsersService, which will fail.

Possible solutions:

1. Add a new resolver to NOTIFICATIONS_ENTITY_RESOLVERS that handles system user
2. Handle system user here in the similar way to how it's done here
3. Other ideas?