Return a failed Google callback to sign-in and correct stale statements

A Google callback that could not finish ended in an unhandled 500: a
visitor who declined on Google's consent screen, a callback whose state
cookie had expired, and a direct request to the callback path all got an
empty error page. The account API document already promised a redirect
to /login?google=failed for a failed callback; the handler of the Google
scheme now does that through OnRemoteFailure. Three new tests cover a
callback without state, a declined consent and a forged state.

Three statements no longer matched the sites:

- the bug report template still named the catalog paths without the
  language and the build system; they are /langs/tala/<Publisher>/<Name>/
  and /dslplugins/tazel/<Publisher>/<Name>/;
- the operations guide placed Tala content on xsharp-lang.xyz, which only
  redirects to tala.run now;
- the architecture guide described the ViGet routing before the catalogs
  per language and build system: it now gives the rewrites of /langs/tala
  and /dslplugins/tazel, the redirects of /langs and /dslplugins, three
  indexed ViGet pages and a sitemap with the home page and both catalogs.

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