[FIX] web_planner: modal body height in IE11

Credits go to qsm for the fix.

addons/web_planner/static/src/less/web_planner_common.less

@@ -176,10 +176,10 @@
                 padding-bottom: 30px;
                 overflow: auto;
 
-                -ms-flex: 1 0 0;
-                -moz-flex: 1 0 0;
-                -webkit-flex: 1 0 0;
-                flex: 1 0 0;
+                -ms-flex: 1 1 auto;
+                -moz-flex: 1 1 auto;
+                -webkit-flex: 1 1 auto;
+                flex: 1 1 auto;
 
                 .o_planner_content_wrapper {
                     position: absolute;