hide welcome column on group boards

there was discussion on Slack to not show the Welcome screen for Group Boards. Reduces the chance of duplicate todo & doing labels from Group and Project

Rather than some group-specific changes to boards_bundle.js and boards_stoe, I think the easiest way is to set issue_board_welcome_hidden

/cc @felipe_artur

Thanks =)

cc @victorwu @smcgivern

Sounds good thanks @psimyn . I think that's great if it's a quick change.