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/core/gr-main-header/gr-main-header.html

@@ -78,6 +78,9 @@ limitations under the License.
         flex: 1;
         justify-content: flex-end;
       }
+      .rightItems gr-endpoint-decorator:not(:empty) {
+        margin-left: 1em;
+      }
       gr-search-bar {
         flex-grow: 1;
         margin-left: .5em;
@@ -121,6 +124,7 @@ limitations under the License.
         }
         gr-search-bar,
         .browse,
+        .rightItems .hideOnMobile,
         .links > li.hideOnMobile {
           display: none;
         }
@@ -159,6 +163,9 @@ limitations under the License.
       </ul>
       <div class="rightItems">
         <gr-search-bar value="{{searchQuery}}" role="search"></gr-search-bar>
+        <gr-endpoint-decorator
+            class="hideOnMobile"
+            name="header-browse-source"></gr-endpoint-decorator>
         <div class="accountContainer" id="accountContainer">
           <a class="loginButton" href$="[[_loginURL]]">Sign in</a>
           <gr-account-dropdown account="[[_account]]"></gr-account-dropdown>

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>