Position the QR code within the modal

apps/explorer_web/assets/css/app.scss

@@ -61,6 +61,7 @@ $fa-font-path: "~@fortawesome/fontawesome-free/webfonts";
 @import "components/filter";
 @import "components/button";
 @import "components/table";
+@import "components/qr-code";
 
 :export {
   primary: $primary;

apps/explorer_web/assets/css/components/_qr-code.scss

@@ -0,0 +1,4 @@
+.qr-code {
+  display: block;
+  margin: auto;
+}

apps/explorer_web/lib/explorer_web/templates/address/overview.html.eex

@@ -69,7 +69,7 @@
             <button class="btn btn-primary" data-clipboard-text="<%= @address %>">Copy</button>
           </div>
         </div>
-        <%= img_tag(address_qr_code_path(@conn, :index, @conn.assigns.locale, @address), alt: "qr_code") %>
+        <%= img_tag(address_qr_code_path(@conn, :index, @conn.assigns.locale, @address), alt: "qr_code", class: "qr-code") %>
       </div>
       <div class="modal-footer">
         <button type="button" class="btn btn-primary" data-dismiss="modal">Close</button>