Allow the message to grow to fit the screen width

Setting the 'flex-grow: 1' style property allows the message to grow
horizontally to fill the space not used by the Dismiss button.

Change-Id: Ia3686e28ea4e36fb5faa38a3960f3d8838062eeb
Release-Notes: Message can grow horizontally to fill space not used by the Dismiss button
diff --git a/gr-messageoftheday/gr-messageoftheday-banner_html.js b/gr-messageoftheday/gr-messageoftheday-banner_html.js
index c0230bd..d4d959c 100644
--- a/gr-messageoftheday/gr-messageoftheday-banner_html.js
+++ b/gr-messageoftheday/gr-messageoftheday-banner_html.js
@@ -24,6 +24,9 @@
     justify-content: space-between;
     padding: 1em;
   }
+  #message {
+    flex-grow: 1;
+  }
 </style>
 <div id="container" hidden$="[[_hidden]]">
   <div id="message"></div>