webserver: cosmetic refactoring
webserver: add support for If-Unmodified-Since HTTP header
webserver: add Cache-Control HTTP header to cacheable responses
webserver: handle Cache-Control: no-cache and Pragma: no-cache
webserver: use defines for HTTP header fields and values
webserver: silence warning
webserver: refactor IsAuthenticated()
Merge pull request #5573 from Montellese/webserver_caching

webserver: improved caching control