New admin option for custom component at top of page

[jessrynkar]

Add new admin option that allows the user to insert a custom component at the top of the page globally (similar to beforeNavLinks / afterNavLinks).

Potential option names:

• topbar
• header
• any other ideas?

PR: #7647

[mikecebul]

Maybe appbar, but I think I prefer header. Too bad it couldn't be Nextjs specific cause you could just use a root layout which we could use for a header, footer, and/or sidebar.