Tag remote Slack users

[pm5]

Currently tagging users (e.g. @pm5) works in a confusing way. On the one hand you could tag a remote Slack user. In the following example both users are local and the author of the message is remote:

This works if the author copies the remote users' name verbatim into the message, because account data is not exchanged between bridged Slack channels.

In this case what the users need is:

1. If the user knows another user by name in one of the bridged channels, the user needs to be able to specify that remote user to make the tag in the message.
2. The tag needs to appear in other bridged channels in a way that readers understand it is for someone in one of the bridged channels.
3. If two different users have the same username but in different Slack channels, users need to be able to distinguish them when writing and reading messages.