Soy index cleanup

Some cleanup on the use of Soy index:
* The canonical-path property of GR-APP is removed because it did not
  initialize at the right time and a top-level global is used instead.
* Only set a top-level global for canonical path if it is not the empty
  string. In this way, the index document does not include an inline
  script when it is not needed.
* Give the index Soy template a more descriptive name. index.html.soy
  was too generic, and the intermediate period would cause problems with
  some internal build tooling.

Bug: Issue 5919
Change-Id: I0bf5964b17cb4f4f9df150b0c6abe8ae1e49f22b

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" canonical-path="/abc/def/ghi"></gr-app>
+    <gr-app id="app"></gr-app>
   </template>
 </test-fixture>
 
@@ -107,9 +107,5 @@ limitations under the License.
         done();
       });
     });
-
-    test('canonical-path', function() {
-      assert.equal(Gerrit.CANONICAL_PATH, '/abc/def/ghi');
-    });
   });
 </script>