Add a single feature flag to control custom admin roles

What does this MR do and why?

Since custom admin roles are under active development, this MR introduces a single WIP feature flag to control the availability of all custom admin roles.

References

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• Closes #515665 (closed)
• Closes #514810 (closed)
• Closes #512831 (closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Create a new admin member role with read_admin_dashboard permission enabled

   admin_member_role = MemberRole.create(name: 'read_admin_subscription', description: 'read_admin_subscription', read_admin_subscription: true)

2. Assign the role to a user. The command to find the user may differ from this example.

   user = User.find_by(username: 'read_admin_subscription')
   Users::UserMemberRole.create(member_role: admin_member_role, user: user)

3. Test that the feature flag works as expected

   > Feature.disable(:custom_admin_roles)
   > User.find_by(username: 'read_admin_subscription').can?(:read_admin_subscription)
   => false
   
   > Feature.enable(:custom_admin_roles)
   > User.find_by(username: 'read_admin_subscription').can?(:read_admin_subscription)
   => true