Bugfix: Channel's user list

Fix channels's user list generation: currently we're not validating if a user coming from grappa already has an account on our platform.

Proposed solution:

• After getting users from grappa, if the user is not already is the channel users list, try fetching the user from our db by username. Consider performance implications of fetch in bulk of one by one.

A couple of specs:

• If user exists in both grappa and db we need to give priority to data from users coming our db