Support serving site-specific logo, favicon, and setting a site title.

Upgrade it to 4.1.3, including dependencies.

Just serve an error on the logout page if there is no valid session,
instead of redirecting to the login workflow.

Fixes issue #8.

Fixes issue #6.

And standardize on a single structure for the gorilla session
map, using the 'data' key for our gob-encoded objects.

Using the new configuration variable 'keystore_enable_groups'.

Likely fixes issue #7 (untested).

Without this field, U2F just isn't working and the accounts with U2F
set up are shown an OTP prompt.

There seems to be no good reason to change group membership when
exchanging tokens, let's try just passing the old one over.

Using CORS-enabled requests in the background.

Allow loading remote images.

Simplifies the device manager code as we don't have to duplicate the
IP/network matching logic there.

Should make the subdivision between apps (idp, sso) more obvious.

Uses the clientutil.Backend abstraction for the keystore client API.

It left the Prometheus instrumentation in server/http.go, while
it is already provided by the ai3/go-common package.

Panic on short reads and other errors.