Allowing underscores in channel names. Added conversion of some slack channel names into valid mattermost names.

model/team.go

@@ -158,7 +158,7 @@ func IsReservedTeamName(s string) bool {
 
 func IsValidTeamName(s string) bool {
 
-	if !IsValidAlphaNum(s) {
+	if !IsValidAlphaNum(s, false) {
 		return false
 	}