Add tooltips to check links

We cannot use the Polymer based TooltipMixin (which too complicated
anyway and we are happy getting rid of it at some point), because the
tooltip is being used in a LitElement.

So we are adding a new dependency on paper-tooltip.

Change-Id: Ifd0c88c214f38fcc5822a4f50f5aa624c34a9ec5
diff --git a/polygerrit-ui/app/elements/checks/gr-checks-results.ts b/polygerrit-ui/app/elements/checks/gr-checks-results.ts
index ef2430c..c0cfca6 100644
--- a/polygerrit-ui/app/elements/checks/gr-checks-results.ts
+++ b/polygerrit-ui/app/elements/checks/gr-checks-results.ts
@@ -24,6 +24,7 @@
   query,
 } from 'lit-element';
 import {GrLitElement} from '../lit/gr-lit-element';
+import '@polymer/paper-tooltip/paper-tooltip';
 import {
   Category,
   CheckRun,
@@ -229,6 +230,7 @@
   }
 
   renderLink(link: Link) {
+    const tooltipText = link.tooltip ?? 'Link to details';
     return html`
       <a href="${link.url}" target="_blank">
         <iron-icon
@@ -236,6 +238,7 @@
           class="launch"
           icon="gr-icons:launch"
         ></iron-icon>
+        <paper-tooltip offset="5">${tooltipText}</paper-tooltip>
       </a>
     `;
   }