Fix the "direct_channel_members" fields returned by channels resource

[RomaricMourgues]

We should always get a proper direct_channel_members in channels:

If channel is direct: direct_channel_members contains the users of the direct channel

Else: direct_channel_members is an empty list

[chamerling]

After discussion, we choose to keep members array for every channel to be consistant. We will do the switch on the frontend based on the channel type (public, private, direct)