[NEW] Mention notification
Closes #12303
When user mentioned in the private channel and he is not inside the room already he gets silent message with information that user is not in the channel and provides option to invite if current user has permission.

Before invite

After invite

@lomboboo does this behavior works on public channels too?
Our current behaviour for public channels is to automatically subscribe mentioned users. There is an existing issue asking to be able to disable that behaviour.
There is also the /invite command, that automatically subscribe users to any channel.
Rocket.Chat currently allows users to be mentioned even when they are not in the channel because it is used as a "send contact" feature.
@rodrigok, as Hudell mentioned it targets only private channels because public have force invite implemented.
We need to remove the force invite, if you can do this as part of this PR it would be great.
Can you make it so that the Invite button disappears after the user joins? Or make it go away on refresh like we do with the "invalid command" messages.
@engelgabriel , yes I'll do it. So that would be the flow? I mean removing force invite only on private channels? And after invite user will get some sort of a message with button to join?
@Hudell I think it would be better to remove whole message. I'll do it in this PR as well
So I've modified this feature to remove force invite and send accept invitation message first, then user can decide if he wants to join this channel.
First gif shows how mention notification works with new logic:

Second is about invite users from flex-tab and slash commands:

@thiagosanchz could you please take a look at the design of this feature?
➤ [email protected] commented:
Abstract links: Channel CTA: https://share.goabstract.com/ba9754d4-b329-4db8-ba3e-aa301963a2c9 Pivate chat CTA: https://share.goabstract.com/1b3a8940-05e1-45a0-bc74-eb54fb8f08cd
Hi! Why was this never merged? This would be a great feature!
Hi, when is this PR getting merged to rocketchat.?