get card drag/drop working for empty swimlanes

fixes this issues:
- #4033
- #4032
- #3415
- #1887

related to this discussion:
- #4868

client/components/lists/list.css

@@ -125,6 +125,8 @@
 .list-body .minicards {
   flex-grow: 1;
   flex-shrink: 0;
+  /** get card drag/drop working for empty swimlanes */
+  min-height: 32px;
 }
 .list-body .minicards form {
   margin-bottom: 9px;