Move setting session.mtime from dbapi to web session manager clean_session was broken since the switch away from dbapi on the web side, since session.mtime wasn't being set anywhere.

[web] whitespace cleanup in http_headers.py

[web] implement cross origin resource sharing (CORS) (closes #2491768) Partial implementation that is enough to get started but leaves out some of the advanced features like caching and non-simple methods and headers.