Session timeout modal appears underneath the group invitation modal

[danielrafailov]

Problem

If the group invitation modal (invite_dialog) is open when the session times out, the session timeout / expired modal opens underneath the invite modal. The timeout message is hard to see or interact with until the invite modal is closed. I don't think that this was introduced by the invite-by-email work (#8106) - it looks like a general modal stacking / z-index issue when two ModalMarkus dialogs are open at once.

Steps to reproduce

1. Log in as a student on an assignment that allows groups.
2. Open the Invite modal on the assignment page.
3. Leave the modal open until the session expires (or otherwise trigger the session timeout / expired modal while the invite modal is still open).
4. Observe that the timeout modal appears behind the invite modal.

Expected behaviour

The session timeout / expired modal should appear on top of any other open modal (including the invite modal), so the user can acknowledge it and re-authenticate.

Actual behaviour

The timeout modal is rendered underneath the invite modal.

Screenshot

Notes

• Related dialogs: #invite_dialog, #session_expired_dialog / #timeout_imminent_dialog (via ModalMarkus in layouts.js).
• Possibly affects other pages whenever a page-specific is open at timeout, not only group invites.