Add an extension point to show a small banner next to the search bar

A plugin or a site theme (gerrit-theme.html) may register a custom
Element to be inserted into this endpoint.

``` js
Gerrit.install(plugin => {
  plugin.registerCustomComponent(
      'header-small-banner', 'gt-header-small-banner', {replace: true});
});
```

``` html
<dom-module id="gt-header-small-banner">
  <template>
    <a href="http://somecode.com/">Bug Tracker</a>
  </template>
  <script>
    Polymer({is: 'gt-header-small-banner'});
  </script>
</dom-module>
````

Change-Id: Ib42550271bae27089978c5cbd81642361acdaaa2
(cherry picked from commit 09fa976ebcfd5a05b681099ea5633701cb3274d5)

polygerrit-ui/app/elements/core/gr-main-header/gr-main-header.html

@@ -189,6 +189,9 @@ limitations under the License.
         </template>
       </ul>
       <div class="rightItems">
+        <gr-endpoint-decorator
+            class="hideOnMobile"
+            name="header-small-banner"></gr-endpoint-decorator>
         <gr-smart-search
             id="search"
             search-query="{{searchQuery}}"></gr-smart-search>