Bug 1619815 - Display a "What's New" link next to the "Download Update" button in About dialog. r=mkmelin

The "Release Notes" link always points to the current running version. This links
to the update's release notes for beta and release channels. (Daily does
not have any release notes to link to.)

Differential Revision: https://phabricator.services.mozilla.com/D137847

mail/base/content/aboutDialog-appUpdater.js

@@ -236,6 +236,12 @@ appUpdater.prototype = {
           let month = buildID.slice(4, 6);
           let day = buildID.slice(6, 8);
           updateVersion += ` (${year}-${month}-${day})`;
+        } else {
+          let updateNotesLink = document.getElementById("updateNotes");
+          if (updateNotesLink) {
+            updateNotesLink.href = gAppUpdater.update.detailsURL;
+            updateNotesLink.hidden = false;
+          }
         }
         button.textContent = this.bundle.formatStringFromName(
           "update.downloadAndInstallButton.label",

mail/base/content/aboutDialog.css

@@ -133,6 +133,10 @@
   text-decoration: var(--link-decoration);
 }
 
+#updateNotes {
+  margin-inline-start: 5px;
+}
+
 #currentChannel {
   margin: 0;
   padding: 0;

mail/base/content/aboutDialog.xhtml

@@ -31,6 +31,9 @@
         role="dialog"
         aria-describedby="version distribution distributionId currentChannelText communityDesc contributeDesc trademark"
         >
+  <linkset>
+    <html:link rel="localization" href="messenger/aboutDialog.ftl"/>
+  </linkset>
 
   <script src="chrome://messenger/content/aboutDialog.js"/>
 #ifdef MOZ_UPDATER
@@ -64,6 +67,8 @@
                 <html:button id="downloadAndInstallButton"
                              onclick="gAppUpdater.startDownload();">
                 </html:button>
+                <html:a id="updateNotes" class="text-link download-link" hidden="hidden"
+                        data-l10n-id="about-update-whats-new"></html:a>
               </html:div>
               <html:div id="apply" class="update-deck-container">
                 <html:button id="updateButton"

mail/locales/en-US/messenger/aboutDialog.ftl

@@ -0,0 +1,5 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+about-update-whats-new = What’s New