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
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
index 20f455a..168ec57 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.js
@@ -449,7 +449,7 @@
     },
 
     _paramsChanged(value) {
-      if (value.view != this.tagName.toLowerCase()) { return; }
+      if (value.view !== Gerrit.Nav.View.DIFF) { return; }
 
       this._loadHash(this.params.hash);