StudioContainerWithNestedXBlocksMixin - default css for buttons

[e-kolpakov]

StudioContainerWithNestedXBlocksMixin requires descendant block to provide it's own css rules for buttons, so if it gails to do that buttons look like this:

The css rules to avoid that would likely be simple: https://github.com/open-craft/xblock-group-project-v2/blob/master/group_project_v2/public/css/group_project_edit.css#L2-L14

[feanil]

Closing old issues before archiving the repository.