use seperate xml designs for Items (+use viewbinding)

[mahibi]

UserItem.java uses 2 different xml designs which adds unnecessary complexity. It's then used in ConversationInforController and ContactsController.

There should be seperate Items for each use case. This is also required to switch from Butterknife to viewbinding.

assigned @AndyScherzinger as we discussed it while talking about #1668

[mahibi]

futhermore rv_item_conversation_info_participant.xml is used for MentionAutocomplteItem and UserItem but this might be okay..