Issue #126 - Add warning message to Playback component if disabled

Update the Playback component to display a warning banner if no valid
time ranges were returned from the backend. This can either be because
there's no valid time ranges of data in the database or (more likely)
because the database connection isn't configured or failed.

ait/gui/static/js/ait/gui/Playback.js

@@ -54,11 +54,20 @@ const Playback = {
         // Display time ranges available
         let range = m('div', {class: 'form-group'}, [
             m('label', 'Time ranges available'),
-            this._range.map(function(i) {
-                return m('div', i[0] + ': ' + i[1] + ' to ' + i[2])
-            })
+            m('div', {'class': 'alert alert-warning'},
+                'No time ranges found. Is your database connection configured?'
+            )
         ])
 
+        if (this._range.length > 0) {
+            range = m('div', {class: 'form-group'}, [
+                m('label', 'Time ranges available'),
+                this._range.map(function(i) {
+                    return m('div', i[0] + ': ' + i[1] + ' to ' + i[2])
+                })
+            ])
+        }
+
         // Packet select drop down menu
         let packets = m('div', {class: 'form-group col-xs-3'}, [
             m('label', 'Telemetry packet:'),
@@ -277,4 +286,4 @@ const Playback = {
 
 
 export default Playback
-export { Playback }
+export { Playback }