`api.storage` kept one `plugin:<id>:` namespace for everyone signed in
to the browser and left it behind after signing out, so a plugin read
what it had stored for the previous account, such as quick-notes'
notes on someone else's mail.
Keys now carry the account (`plugin:<id>:@<account>:<key>`). Signing an
account out or removing it deletes its keys, and a full sign-out
deletes all plugin storage. Keys written before this change are handed
to the first account that uses the plugin. Uninstalling a plugin still
removes everything under its prefix.
