*: rename internally used "state" form value to "req"

"state" means something specific to OAuth2 and SAML so we don't
want to confuse developers who are working on this.

Also don't use "session" which could easily be confused with HTTP
cookies.

web/templates/login.html

@@ -6,7 +6,7 @@
   <div>
     {{ range $c := .Connectors }}
       <div class="form-row">
-        <a href="{{ $c.URL }}?state={{ $.State }}" target="_self">
+        <a href="{{ $c.URL }}?req={{ $.AuthReqID }}" target="_self">
           <button class="btn btn-provider">
             <span class="btn-icon btn-icon-{{ $c.ID }}"></span>
             <span class="btn-text">Log in with {{ $c.Name }}</span>