Handle message timestamp taps with Nav API

Very similar to Ib045b8f. Removes more references to page.show, adds
'messageHash' field to getUrlForChange, and utilizes it in the change
view.

Change-Id: I496d731ceeba57843d617fd5b978cd377ea39787
diff --git a/polygerrit-ui/app/elements/change/gr-message/gr-message.html b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
index 19b0716b..32c4c1f 100644
--- a/polygerrit-ui/app/elements/change/gr-message/gr-message.html
+++ b/polygerrit-ui/app/elements/change/gr-message/gr-message.html
@@ -132,9 +132,12 @@
         right: var(--default-horizontal-margin);
         top: 10px;
       }
-      .date {
+      span.date {
         color: var(--deemphasized-text-color);
       }
+      span.date:hover {
+        text-decoration: underline;
+      }
       .dateContainer iron-icon {
         cursor: pointer;
       }
@@ -227,12 +230,12 @@
             </span>
           </template>
           <template is="dom-if" if="[[message.id]]">
-            <a class="date" href$="[[_computeMessageHash(message)]]" on-tap="_handleLinkTap">
+            <span class="date" on-tap="_handleAnchorTap">
               <gr-date-formatter
                   has-tooltip
                   show-date-and-time
                   date-str="[[message.date]]"></gr-date-formatter>
-            </a>
+            </span>
           </template>
           <iron-icon
               id="expandToggle"