Add more fields to Gerrit.Nav.View and utilize in app

Previously, the app relied on string matching of the view tag names in
order to perform view-based actions (e.g. routing).

This change refactors those instances to utilise the Gerrit.Nav.View
enum.

Bug: Issue 6708
Change-Id: If0212fde93e0167e3207af19006beee1a602df60

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html

diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
index 2fc05f7..1503f93f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.html
@@ -469,7 +469,7 @@
 
       element._loggedIn = true;
       element.params = {
-        view: 'gr-diff-view',
+        view: Gerrit.Nav.View.DIFF,
         changeNum: '42',
         patchNum: '2',
         basePatchNum: '1',
@@ -493,7 +493,7 @@
       });
     });
 
-    test('hash is determined from parmas', done => {
+    test('hash is determined from params', done => {
       stub('gr-rest-api-interface', {
         getDiffComments() { return Promise.resolve({}); },
       });
@@ -502,7 +502,7 @@
 
       element._loggedIn = true;
       element.params = {
-        view: 'gr-diff-view',
+        view: Gerrit.Nav.View.DIFF,
         changeNum: '42',
         patchNum: '2',
         basePatchNum: '1',