Refactor the login handler

See merge request !6

Session handling no longer depends on gorilla/sessions.

Drop gorilla/sessions in favor of using gorilla/securecookie
directly (we use a single cookie anyway). Since securecookie already
has its own expiration timestamp, we can drop some stuff from httputil
as well.

There is no need for the complex gorilla/sessions machinery for what
is basically a single cookie, so we switch to using
gorilla/securecookie directly.

There is no point in using the complex gorilla/sessions machinery for
storing a simple long-term cookie, just use gorilla/securecookie directly.

If we don't, they will trigger the login handler and invalidate the
current session (if any), which prevents the user from being able to
log in.

Being consistent across runs avoids generating spurious git changes.

Fix a bug where global variables were missing.

Remove the build-time dependency on Python, the sri_map is now
generated via a small Go script.

The login handler is now a simpler, standalone http.Handler
wrapper. The separation between the SSO application and the login
handler is now fairly complete.

The login handler no longer forces the user to a specific workflow via
session cookies, but it works on a request-by-request basis instead,
which makes the "back" button works as expected (allowing the user to
bail out of a broken 2FA process, for example).

Session handling has been simplified as well: there is a single
session for authentication and login state, which should remove the
opportunity for session synchronization errors.

And fix a bug with inline XML descriptors.

One can now restrict providers to specific SSO groups.

Add methods to the httpsso package to retrieve authentication info
from the current session (by passing an *http.Request object).

Vertically align SiteLogo with form

See merge request !5

Fix documentation and add missing logo if configured

See merge request !4

Temporary workaround if we do not want to maintain a whitelist.