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.

<!-- dollarId: issue.index,v 1.2 2001/07/29 04:07:37 richard Exp dollar-->

<tal:block metal:use-macro="templates/page/macros/icing">

<title metal:fill-slot="head_title" i18n:translate="" >
 About this Tracker
</title>
<tal:block metal:fill-slot="body_title" i18n:translate="">
  About this Tracker
</tal:block>

<div class="content" metal:fill-slot="content">

 <span tal:condition="not:python:request.user.hasRole('Admin')"
	tal:omit-tag="python:1" i18n:translate="">
    Please login with your username and password to find out about
    this tracker.
  </span>

  <div tal:condition="python:request.user.hasRole('Admin')"
       tal:omit-tag="python:1" i18n:translate="">
    <div tal:replace="structure python:utils.AboutPage(db)"></div>
  </div>
</div>

</tal:block>