Allow tagging everyone in a group #540

Just that: using something like @all or @everyone, that would send a targetted notification to all the members on a group.

Looks like nicegram already has something like this.

I know Telegram's servers do not allow tagging more than 50 people on a single message and that there are some bots that could do that job, or pinning a message as an alternative. I don't know if Forkgram's team has a strong opinion about this feature.