Set minimal width of input in ch

Mixing width in em and ch for the number input creates an overflow on the
group.

issue9706
review318481002

src/sao.less

@@ -516,15 +516,15 @@ img.icon {
     .form-multiselection, .form-url, .form-email, .form-callto, .form-sip,
     .form-pyson {
         input, select {
-            min-width: 8em;
+            min-width: 8ch;
         }
     }
     .form-many2one, .form-one2one, .form-reference, form-binary {
         select {
-            min-width: 8em;
+            min-width: 8ch;
         }
         input {
-            min-width: 12em;
+            min-width: 12ch;
         }
     }