fix(web): issue2551356. Add etag header for not-modified (304) request. When a 304 is returned to a conditional request for a static file, print an ETag for the response. ETag was always sent with a 200 response. This also adds initial support for if-none-match conditional requests for static files. Changes: Refactors the if-modified-since code out to a method. It moves a file stat call from serve_static_file to _serve_file so that an etag can be generated by both serve_static_file and serve_file which call _serve_file. Tests added. This does not test the codepath where serve_file pulls content from the database rather than from a local file on disk. Test mocking _serve_file changed to account for 5th argument to serve_file BREAKING CHANGE: function signature for client.py-Client::_serve_file() now has 5 not 4 parameters (added etag param). Since this is a "hidden" method I am not too worried about it.

Other Docs
==========

.. toctree::
   :maxdepth: 2

   docs/upgrading-history

   docs/tracker_templates

   Design Overview <docs/overview>
   Design (original) <docs/design>

   Software Carpentry and Short Papers <docs/sc>

   docs/developers

   Notes about the MySQL Database backend <docs/mysql>
   Notes about the PostgreSQL Database backend <docs/postgresql>

   Richard Jones implementation notes <docs/implementation>
   docs/security-history