dashboard: log out of this browser, and say what can't be

The dashboard had no way to log out; only the login page did.

It now shows the user beside the connection, and "log out" where the
page got in by its session: that ends this browser's session (DELETE
/auth/session), other sessions of the user going on, and opens the
login page. Where it got in by what it can't end, the browser's
password or the proxy's, it says so instead (the token answer now
names `login`, how the user proved it). After logging out, a page
still let in by the browser's password says so too.

A list of a user's sessions, to end one by one, is in future-work.md.

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