MM-32133 shared channel username collisions (#17347)

Support for handling username collisions between remote clusters. Users belonging to remote clusters have their username changed to include the remote name e.g. wiggin becomes wiggin:mattermost.

@mentions are also modified so the munged username is replaced with the original username when the post is sync'd with the remote the user belongs to.

When adding remote users:
- append the remote name to the username with colon separator
- append the remote name to the email address with colon separator
- store the original username and email address in user props
- when resolving @mentions replace with the stored original username

model/config.go

@@ -19,6 +19,7 @@ import (
 	"time"
 
 	"github.com/mattermost/ldap"
+
 	"github.com/mattermost/mattermost-server/v5/shared/filestore"
 	"github.com/mattermost/mattermost-server/v5/shared/mlog"
 )