[NEW] Mark message as unread

[diegolmello]

Similar to #709, we should have an option to mark the message as unread.

You need to:

• Add "Mark as unread" option on MessageActions (fired on message long press)
• Call subscriptions.unread with firstUnreadMessage._id: https://rocket.chat/docs/developer-guides/rest-api/subscriptions/unread/
• Go to rooms list (otherwise you'll mark the rooms as read again)

[diegolmello]

@weijia-yu Can you work on this?

[weijia-yu]

@diegolmello yes

[peterchiuglg]

it doesn't work