feat(search): keyboard-accessible search button (#1331)

* Use a `<button>` for search

* Fix search button styles to match preexisting styling

* Remove additional native button properties.

* Invoke search button on click or keyboard.

* Reorganize search button DOM hierarchy

* Restore focus to the search button when exiting the search overlay

* Run prettier on Search.tsx

quartz/components/Search.tsx

@@ -19,24 +19,16 @@ export default ((userOpts?: Partial<SearchOptions>) => {
     const searchPlaceholder = i18n(cfg.locale).components.search.searchBarPlaceholder
     return (
       <div class={classNames(displayClass, "search")}>
-        <div id="search-icon">
+        <button class="search-button" id="search-button">
           <p>{i18n(cfg.locale).components.search.title}</p>
-          <div></div>
-          <svg
-            tabIndex={0}
-            aria-labelledby="title desc"
-            role="img"
-            xmlns="http://www.w3.org/2000/svg"
-            viewBox="0 0 19.9 19.7"
-          >
-            <title id="title">Search</title>
-            <desc id="desc">Search</desc>
+          <svg role="img" xmlns="http://www.w3.org/2000/svg" viewBox="0 0 19.9 19.7">
+            <title>Search</title>
             <g class="search-path" fill="none">
               <path stroke-linecap="square" d="M18.5 18.3l-5.4-5.4" />
               <circle cx="8" cy="8" r="7" />
             </g>
           </svg>
-        </div>
+        </button>
         <div id="search-container">
           <div id="search-space">
             <input