Update external link icon in groups menu

The icon is from the Hypothicons set at https://hypothicons.netlify.com

src/images/icons/external.svg

+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16" aria-hidden="true" focusable="false" class="Icon Icon--external"><g fill-rule="evenodd"><path fill="none" fill-rule="evenodd" stroke="currentColor" stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M7 3h6v6m-1-5l-9 9 9-9z"></path><rect fill="none" stroke="none" x="0" y="0" width="16" height="16"></rect></g></svg>
\ No newline at end of file

src/sidebar/components/group-list-item.js

@@ -87,7 +87,7 @@ function GroupListItem({
               <li>
                 <MenuItem
                   href={activityUrl}
-                  icon="share"
+                  icon="external"
                   isSubmenuItem={true}
                   label="View group activity"
                 />

src/sidebar/components/svg-icon.js

@@ -11,6 +11,7 @@ const icons = {
   'expand-menu': require('../../images/icons/expand-menu.svg'),
   copy: require('../../images/icons/copy.svg'),
   cursor: require('../../images/icons/cursor.svg'),
+  external: require('../../images/icons/external.svg'),
   help: require('../../images/icons/help.svg'),
   leave: require('../../images/icons/leave.svg'),
   refresh: require('../../images/icons/refresh.svg'),