[ ] Account settings show 'open-platform.sh: linked as <login>' or 'not linked', with Link and Unlink. Link records the edge login the current request carries against the signed-in account. Unlink removes it. Both are same-origin POSTs.
[ ] An edge sign-in finds the account by its link first, then by name for accounts the edge made before this landed.
[ ] Tests: link, then an edge sign-in reaches the linked account. After unlink, it no longer does. One edge login cannot be linked to two accounts.
Part 4 of #668, after the account rule.
Done looks like: