Add Lunr search UI integration (#580)

- Add search input field to toolbar (conditionally rendered when
  SITE_SEARCH_PROVIDER is set)
- Load lunr.js, search-ui.js, and search-index.js scripts
- Add maximum-scale=1 to viewport meta tag for better mobile behavior

Refs: boostorg/boostlook#142

antora-ui/src/partials/footer-scripts.hbs

@@ -1,3 +1,9 @@
 <script id="site-script" src="{{{uiRootPath}}}/js/site.js" data-ui-root-path="{{{uiRootPath}}}"></script>
 <script async src="{{{uiRootPath}}}/js/vendor/highlight.js"></script>
 <script async src="{{{uiRootPath}}}/js/vendor/tabs.js" data-sync-storage-key="preferred-tab"></script>
+{{#if env.SITE_SEARCH_PROVIDER}}
+<script async src="{{{uiRootPath}}}/js/vendor/lunr.js"></script>
+<script async src="{{{uiRootPath}}}/js/search-ui.js" id="search-ui-script" data-site-root-path="{{{siteRootPath}}}"
+  data-ui-root-path="{{{uiRootPath}}}"></script>
+<script async src="{{{siteRootPath}}}/search-index.js"></script>
+{{/if}}

antora-ui/src/partials/head-prelude.hbs

@@ -1,2 +1,2 @@
     <meta charset="utf-8">
-    <meta name="viewport" content="width=device-width,initial-scale=1">
+    <meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1">

antora-ui/src/partials/header-content.hbs

@@ -1,6 +1,5 @@
  <div id="header">
-  {{#if (and (eq page.component.latest.asciidoc.attributes.remove-sidenav undefined) (eq page.attributes.remove-sidenav
-  undefined))}}
+    {{#if (and (eq page.component.latest.asciidoc.attributes.remove-sidenav undefined) (eq page.attributes.remove-sidenav undefined))}}
       {{> nav}}
     {{/if}}
   </div>

antora-ui/src/partials/toolbar.hbs

@@ -1,5 +1,12 @@
 <div class="toolbar" role="navigation">
 {{> nav-toggle}}
 {{> breadcrumbs}}
+{{#if env.SITE_SEARCH_PROVIDER}}
+  <div class="search-container">
+    <div id="search-field" class="field has-filter">
+      <input id="search-input" type="text" placeholder="Search the docs" {{#if page.home}} autofocus{{/if}}>
+    </div>
+  </div>
+{{/if}}
 {{> pagination-spirit}}
 </div>