A full sign-out emptied the stores but left the accounts' traces in the
browser for the next person at the computer: search history, open tabs
with message subjects, Files favourites and recent files, calendar
alert acknowledgements, and attachment bytes a plugin had staged for an
upload that never started. Other open tabs kept showing the signed-out
accounts' mail, and the boot snapshot of the mailbox list was restored
for whichever account happened to be active next.
- A full sign-out now removes those keys and the staged files, and
tells the other tabs through a `storage` event so they leave for the
login page.
- The boot snapshot names its account and is only restored for it.
- The login form only remembers the username for suggestions when the
user did not decline "remember me".
Device preferences and `template-storage` stay: with settings sync off
they exist only on this browser.