Do not change the session-stored subsite, if session is not enabled.

This causes issues with Security::findAnAdmistrator which incorrectly
forces the current session-stored subsite to 0 - it uses
Subsite::currentSubsiteID before the session support is enabled, and
hence obtains wrong value.
Merge pull request #126 from mateusz/session-fix

Do not change the session-stored subsite, if session is not enabled.