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

store/sqlstore/remote_cluster_store.go

@@ -22,9 +22,10 @@ func newSqlRemoteClusterStore(sqlStore *SqlStore) store.RemoteClusterStore {
 	s := &sqlRemoteClusterStore{sqlStore}
 
 	for _, db := range sqlStore.GetAllConns() {
-		table := db.AddTableWithName(model.RemoteCluster{}, "RemoteClusters").SetKeys(false, "RemoteId")
+		table := db.AddTableWithName(model.RemoteCluster{}, "RemoteClusters").SetKeys(false, "RemoteId", "Name")
 		table.ColMap("RemoteId").SetMaxSize(26)
 		table.ColMap("RemoteTeamId").SetMaxSize(26)
+		table.ColMap("Name").SetMaxSize(64)
 		table.ColMap("DisplayName").SetMaxSize(64)
 		table.ColMap("SiteURL").SetMaxSize(512)
 		table.ColMap("Token").SetMaxSize(26)