Add support for PDFs

lib/ui/tiles/post_tile.dart

@@ -177,6 +177,10 @@ class _PostTileState extends State<PostTile> {
         postContent = _buildPostWebLink();
         break;
 
+      case PostKind.PDF:
+        postContent = _buildPostPDF();
+        break;
+
       case PostKind.COUNTDOWN:
         postContent = _buildCountDownTimer();
         break;
@@ -325,6 +329,16 @@ class _PostTileState extends State<PostTile> {
     );
   }
 
+  Widget _buildPostPDF() {
+    return RaisedButton.icon(
+      onPressed: () {
+        launch(widget.post.fileURL);
+      },
+      icon: Icon(Icons.picture_as_pdf),
+      label: Text(tr("PDF")),
+    );
+  }
+
   Widget _buildCountDownTimer() {
     return CountdownWidget(
       startTime: widget.post.timeSent,