Add Administration menu for Group Owners

Problem

1. It's not clear which settings at the parent group level are settings that are enforced and/or inherited throughout all groups and projects since they are all under the standard Settings menu.
2. We will also eventually need a place to put isolation features such as Group Managed Accounts and User Management at the parent group level.
3. We should provide a better feeling to Enterprise customers that they are in control of their own "instance" on GitLab.com.

Proposal

• Add new Administration menu item for Group Owners at the parent group level
• Move SAML SSO under the Administration menu
• Move Usage Quotas under the Administration menu
• Move Billing under the Administration menu
• Remove Restrict access by IP address and Restrict membership by email settings from subgroups, since they are never able to be edited at the subgroup level

Why is this important?

Extending Group Managed Accounts to all groups is our first step towards a better experience on GitLab.com for Enterprises wanting isolation and control over their projects, groups and users and a big part of this is introducing administration controls for their group. Introducing this new menu and cleaning up menu items allows us to iterate in the future by further including more admin functions and therefore empowering our Enterprise customers, and adds more emphasis on SAML SSO + GMA as a new feature.

Related Epics and Issues

• &4419
• &4257 (closed)
• #209020 (closed)
• &2885 (closed)