Center checkbox in editable tree

For the auto margin to center the input, it must be displayed as block without
float.

Closes #14942
(grafted from 99d59e3b76a7f11c65adea228345fe24571ed510)

--HG--
branch : 7.0

sao/src/sao.less

@@ -537,9 +537,11 @@ img.icon {
         width: 100%;
 
         input[type='checkbox'] {
-            margin-top: 0;
+            display: block;
+            float: unset;
             margin-left: auto;
             margin-right: auto;
+            margin-top: 0;
         }
 
         > .input-group {