Allow for overriding the "Restricted visibility levels" at the group layer
Description
We are currently facing a problem where Group owners (or any other access level) cannot create project in their private groups due to the global setting under Admin Area -> Settings -> Visibility and Access Control -> Restricted Visibility, which only allows GitLab Administrators to create private Groups, Projects and Snippets. The below error shows that a user has tried to create a private project in their private group, with the private access level set (being the only available setting), but they can't create private projects as a non-admin, and as our default setting for creating projects is set to internal, they are presented with this error. The error itself is a little misleading, so if the message was changed it would be a bit more useful to the users.
(Please note this screenshot is from 12.0, in 12.1, the UI has changed to remove the non-selectable visibility options).
Proposal
I would like to propose in the first instance to change the message to something along the lines of "Only Administrators are allowed to Create (Private/Internal/Public) (Projects/Groups/Snippets)". And then eventually, it would be good to be able to override the instance-wide setting for restricting visibility levels, using that setting as more of a default. This way, Group owners can then create projects or sub-groups within those groups that they own.