Add an extension point for Source Browser link

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-browse-source', 'gt-header-source', {replace: true});
});
```

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

Feature: Issue 7120
Change-Id: Ib4b133de755c16ae32e464e7c4352310f29ef5b0

polygerrit-ui/app/elements/plugins/gr-endpoint-decorator/gr-endpoint-decorator.html

@@ -18,7 +18,7 @@ limitations under the License.
 <link rel="import" href="../../shared/gr-js-api-interface/gr-js-api-interface.html">
 
 <dom-module id="gr-endpoint-decorator">
-  <template>
+  <template strip-whitespace>
     <content></content>
   </template>
   <script src="gr-endpoint-decorator.js"></script>