Fix logic on issue assignment during prepublication checklist

[joemull]

In addition to the logic changes specified in the bug description, I've made the primary issue visible on the modal:

I've also added "Primary" before the primary issue in the OLH and material theme sidebars for consistency, since we prepend the name of the issue type (e.g. "Collection" or "Issue") for subsequent issues in the issue list).

[joemull]

Closes #3041.