Disable User Invitations on Self-Managed

Background

There is a lot of interest on Ability to disable user invitations for Enterpr... (#19618 - closed). Due to the nature of the structure of GitLab.com, the self-managed version of this feature needs to be broken out into its own issue. This is that issue's self managed counterpart.

Note that SAML Group Lock is one way to control memberships, without this feature.

Proposal

Create new admin setting that prevents users from being able to invite other users. Default disabled, but customers can enable this setting. Implement as a cascading setting.

This setting will hide the Invite Member / Invite group buttons in the UI at all levels. It will also prevent this on the API.