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="">Registration in progress - <span i18n:name="tracker"
 tal:replace="config/TRACKER_NAME" /></title>
<span metal:fill-slot="body_title" tal:omit-tag="python:1"
 i18n:translate="">Registration in progress...</span>
<td class="content" metal:fill-slot="content">

<p i18n:translate="">You will shortly receive an email
to confirm your registration. To complete the registration process,
visit the link indicated in the email.
</p>

</td>
</tal:block>