Added aria-label to default.hbs' search button

no issue
- Improve accessibility
- Improve SEO (by improving accessibility)

default.hbs

@@ -51,10 +51,10 @@
             <div class="gh-head-actions">
                 {{#unless @site.members_enabled}}
                     {{^match @custom.navigation_layout "Stacked"}}
-                        <button class="gh-search gh-icon-btn" data-ghost-search>{{> "icons/search"}}</button>
+                        <button class="gh-search gh-icon-btn" aria-label="Search this site" data-ghost-search>{{> "icons/search"}}</button>
                     {{/match}}
                 {{else}}
-                    <button class="gh-search gh-icon-btn" data-ghost-search>{{> "icons/search"}}</button>
+                    <button class="gh-search gh-icon-btn" aria-label="Search this site" data-ghost-search>{{> "icons/search"}}</button>
                     <div class="gh-head-members">
                         {{#unless @member}}
                             {{#unless @site.members_invite_only}}