adding clearsessions to let admins clean out expired sessions
adding link to Django documentation
fixing issues with cleaning sessions, close #12435

This commit add customized filesessionstore.py to replace
django.contrib.sessions.backend.file.py,
see https://code.djangoproject.com/ticket/22938
Once this is fixed filesesionstore.py should be removed in favor of the new
Django version
adjusting formatting
making session cookie parameters configurable

SESSION_EXPIRE_AT_BROWSER_CLOSE and SESSION_COOKIE_AGE
updating help
Merge pull request #2686 from aleksandra-tarkowska/session_engine_clean_out

clear sessions