Move multiple issue boards at group level down to Core / CE

This is basically an extension of gitlab-foss#53811 (closed).

In gitlab-com/www-gitlab-com!29612 (comment 216475160) the down movement has not been done, since that issue and decision was only for projects.

This issue is for groups. Having the ability to create multiple issue boards for groups is the only way that I am aware of to track things at a meta level like on Phabricator for example.