Support `flag-message` channel capability

[petyosi]

Motivation The UI should respect the server configuration when it comes to flagging messages. The capability is passed in flag-message.

Proposed solution Take the flag into account in the message actions logic.

Also, check if the rest of the cans don't need the same treatment. Consult the capabilities table.

Acceptance Criteria Disabling the flag from the server should prevent the flag message action from being displayed.