Serve PolyGerrit index.html from a Soy template

Rather than serving a static file, serve the PolyGerrit index HTML
document from a Soy template. In this way, the path to load PG
dependencies can be safely parameterized in two ways:

* If Gerrit is not running on the root of the domain (e.g. listening on
  https://example.com/my-gerrit/) the path component of the Canonical
  Web URL is used to load PG dependencies.

  Instead of /elements/gr-app.js off the root of the domain it uses
  /my-gerrit/elements/gr-app.js

* If the PolyGerrit static resources are to be served from a CDN rather
  than the Gerrit WAR, the `cdnPath` config in the [gerrit] section can
  be used.

  For example, if the server config says ...

  [gerrit]
    cdnPath = http://my-cdn.com/pg/version/123

  ... then it uses the following style of path for PG dependencies.

  http://my-cdn.com/pg/version/123/my-gerrit/elements/gr-app.js

  If a CDN-path is configured, it supersedes subdirectories appearing
  in the Canonical-Web-URL for this purpose.

Feature: Issue 5845
Change-Id: I2b2d704fe33c90ea2f2a2183fc79897642a48175

polygerrit-ui/app/elements/gr-app_test.html

@@ -27,7 +27,7 @@ limitations under the License.
 
 <test-fixture id="basic">
   <template>
-    <gr-app id="app"></gr-app>
+    <gr-app id="app" canonical-path="/abc/def/ghi"></gr-app>
   </template>
 </test-fixture>
 
@@ -41,6 +41,9 @@ limitations under the License.
       stub('gr-reporting', {
         appStarted: sandbox.stub(),
       });
+      stub('gr-account-dropdown', {
+        _getTopContent: sinon.stub(),
+      });
       stub('gr-rest-api-interface', {
         getAccount: function() { return Promise.resolve({}); },
         getAccountCapabilities: function() { return Promise.resolve({}); },
@@ -99,5 +102,9 @@ limitations under the License.
         done();
       });
     });
+
+    test('canonical-path', function() {
+      assert.equal(Gerrit.CANONICAL_PATH, '/abc/def/ghi');
+    });
   });
 </script>