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.

/*  print.css - MoinMoin Default Styles

Copyright (c) 2001, 2002, 2003 by Juergen Hermann
*/

/* content styles */

html {
	font-family: Times, serif;
	font-size: 12pt;
}

body {
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	margin: 1.5cm;
}

a, a:visited, a.nonexistent, a.badinterwiki {
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	text-decoration: none;
}

a:hover {
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}

.info a {
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}

pre {
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}

a.interwiki:before, a.badinterwiki:before {
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}

a.interwiki img, a.badinterwiki img {
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}

.footnotes div {
	width: 5em;
	border-top: 1pt solid gray;
}

/* user interface styles */

#header, #sidebar, #footer, #timings, #credits, #interwiki, #pagelocation {
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}