Fix column-back-button style for some browsers (#4484)

Use `text-align: unset` instead of `text-align: start` which Edge doesn't support for now.

Also remove default margin on Safari.

app/javascript/styles/components.scss

@@ -1590,8 +1590,9 @@
   flex: 0 0 auto;
   font-size: 16px;
   border: 0;
-  text-align: start;
+  text-align: unset;
   padding: 15px;
+  margin: 0;
   z-index: 3;
 
   &:hover {