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.

<!-- query.item -->
<span tal:condition="context/is_view_ok" tal:replace="structure
      context/renderQueryForm" />
<tal:block tal:condition="not:context/is_view_ok">
  <tal:block metal:use-macro="templates/page/macros/icing">
    <title metal:fill-slot="head_title">You can not view query</title>
    <tal:block metal:fill-slot="body_title">
      You can not view query.
    </tal:block>
    <td class="content" metal:fill-slot="content">
      You are not allowed to view <span tal:content="context/_classname"/>
      with id <span tal:content="context/id"/>
    </td>
  </tal:block>
</tal:block>