Always align on start on xs screen

e5446fb8d4ce reintroduced the rule but using text-center while it is justify-*
that is used for grid layout.

Closes #12807
(grafted from ae247628f5c859df56bbc2494f76cc59f9c152e7)

--HG--
branch : 7.0

sao/src/sao.less

@@ -957,7 +957,8 @@ input.column-boolean {
     }
 
     .xalign-center, .xalign-end {
-        text-align: start;
+        justify-self: start;
+        justify-content: start;
     }
 }