Not possible to use underscores in a group name when MatterMost is enabled
## Description
It appears that it is not possible to use underscore character in a group name when MatterMost integration is enabled.
## Screenshot
## Details
It appears that MatterMost forbids using `_` in a team name, see [related MM code](https://github.com/mattermost/mattermost-server/blob/master/model/team.go#L236).
/cc @victorwu @richardc @zj
issue