Add rel="noopener" to external links

It is recommended to open external links using rel=noopener for both
security and performance reasons.

https://developers.google.com/web/tools/lighthouse/audits/noopener

Change-Id: Ic8fadadbf16f876231345dfdd300a0d2f6a658f2

polygerrit-ui/app/elements/change/gr-commit-info/gr-commit-info.html

@@ -24,7 +24,7 @@ limitations under the License.
       }
     </style>
     <template is="dom-if" if="[[_showWebLink]]">
-      <a target="_blank"
+      <a target="_blank" rel="noopener"
          href$="[[_webLink]]">[[_computeShortHash(commitInfo)]]</a>
     </template>
     <template is="dom-if" if="[[!_showWebLink]]">

polygerrit-ui/app/elements/diff/gr-patch-range-select/gr-patch-range-select.html

@@ -55,7 +55,7 @@ limitations under the License.
     </span>
     <span is="dom-if" if="[[filesWeblinks.meta_a]]" class="filesWeblinks">
       <template is="dom-repeat" items="[[filesWeblinks.meta_a]]" as="weblink">
-        <a target="_blank"
+        <a target="_blank" rel="noopener"
            href$="[[weblink.url]]">[[weblink.name]]</a>
       </template>
     </span>

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

@@ -127,12 +127,13 @@ limitations under the License.
       </div>
     </main>
     <footer role="contentinfo">
-      Powered by <a href="https://www.gerritcodereview.com/" target="_blank">Gerrit Code Review</a>
+      Powered by <a href="https://www.gerritcodereview.com/" rel="noopener"
+      target="_blank">Gerrit Code Review</a>
       ([[_version]])
       |
       <a class="feedback"
           href="https://bugs.chromium.org/p/gerrit/issues/entry?template=PolyGerrit%20Issue"
-          target="_blank">Report PolyGerrit Bug</a>
+          rel="noopener" target="_blank">Report PolyGerrit Bug</a>
       <template is="dom-if" if="[[_computeShowGwtUiLink(_serverConfig)]]">
         |
         <a id="gwtLink" href$="/?polygerrit=0#[[_path]]" rel="external">GWT UI</a>