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.

<tal:block metal:use-macro="templates/page/macros/icing">
<title metal:fill-slot="head_title" i18n:translate="">List of classes - <span
 i18n:name="tracker" tal:replace="config/TRACKER_NAME" /></title>
<span metal:fill-slot="body_title" tal:omit-tag="python:1"
 i18n:translate="">List of classes</span>
<td class="content" metal:fill-slot="content">
<table class="classlist">

<tal:block tal:repeat="cl db/classes">
 <tr>
  <th class="header" colspan="2" align="left">
   <a tal:attributes="href string:${cl/classname}"
      tal:content="python:cl.classname.capitalize()">classname</a>
  </th>
 </tr>
 <tr tal:repeat="prop cl/properties">
  <th tal:content="prop/_name">name</th>
  <td tal:content="prop/_prop">type</td>
 </tr>
</tal:block>

</table>
</td>

</tal:block>