feat($theme-default): disable search box via frontmatter (close: #1287) (#1288)

Author: andezzat

packages/@vuepress/theme-default/components/Navbar.vue

@@ -30,7 +30,7 @@
         v-if="isAlgoliaSearch"
         :options="algolia"
       />
-      <SearchBox v-else-if="$site.themeConfig.search !== false"/>
+      <SearchBox v-else-if="$site.themeConfig.search !== false && $page.frontmatter.search !== false"/>
       <NavLinks class="can-hide"/>
     </div>
   </header>