closes ENG-711
When an Admin is authenticated in Comments-UI we only add moderation options to the displayed comments so we don't need to pre-emptively load the `admin-auth` iframe and make the `/ghost/api/admin/users/me/` request until some comments are actually visible.
- used `state.comments.length` property to defer rendering of the admin auth frame until comments have been fetched (after box is scrolled into view) and the count is > 0
refs https://github.com/TryGhost/Product/issues/3504
- When you are logged in as an admin, but not as a member, no buttons
showed (discovered in new e2e tests)
- Added E2E tests for admin actions