Hygiene: replace header and content divs

- Replace the header div with a header element and the content div with
  a main element. main and header elements are already shivved:
  https://gerrit.wikimedia.org/r/plugins/gitiles/mediawiki/core/+/22dd581f10/resources/lib/html5shiv/html5shiv.js#234.

- Link the wiki header title / logo to the main page. Note: the link
  border around images is already omitted. The link is identical to the
  menu home button.

Bug: T198947
Change-Id: Id312638d86179e75bc670e72e5943f8c00232bbb

includes/skins/MinervaTemplate.php

@@ -295,6 +295,7 @@ class MinervaTemplate extends BaseTemplate {
 			'headelement' => $data[ 'headelement' ],
 			'menuButton' => $data['menuButton'],
 			'headinghtml' => $data['footer-site-heading-html'],
+			'mainPageURL' => Title::newMainPage()->getLocalUrl(),
 			// A button when clicked will submit the form
 			// This is used so that on tablet devices with JS disabled the search button
 			// passes the value of input to the search

includes/skins/minerva.mustache

@@ -7,14 +7,14 @@
 		<div class="banner-container">
 			{{>banners}}
 		</div>
-		<div class="header-container header-chrome">
+		<header class="header-container header-chrome">
 			<form class="header" action="{{wgScript}}" method="get">
 				<div>{{{menuButton}}}</div>
 				<div class="branding-box">
-					<h1>
+					<a href="{{mainPageURL}}">
 						<span>{{{headinghtml}}}</span>
 						<sup>&beta;</sup>
-					</h1>
+					</a>
 				</div>
 				<div class="search-box">
 					<input class="search skin-minerva-search-trigger" type="search" name="search" id="searchInput"
@@ -24,10 +24,10 @@
 				<div>{{{searchButton}}}</div>
 				{{^isAnon}}<div>{{#secondaryButtonData}}{{>secondaryButton}}{{/secondaryButtonData}}</div>{{/isAnon}}
 			</form>
-		</div>
-		<div id="content" class="mw-body">
+		</header>
+		<main id="content" class="mw-body">
 			{{{contenthtml}}}
-		</div>
+		</main>
 		{{>footer}}
 	</div>
 </div>

resources/skins.minerva.base.styles/ui.less

@@ -77,9 +77,12 @@ body {
 	.branding-box {
 		width: auto;
 
-		h1 {
+		h1, // TODO: remove after T198947 HTML changes expire from cache.
+		a {
 			margin-left: 5px;
 			font-size: 1em;
+			text-decoration: none;
+			color: @colorGray2;
 
 			span {
 				line-height: 1;