[FIX] web: fixed opacity of readonly checkboxes

Apparently, browsers decide how they want to style readonly inputs,
and it seems like Firefox is greying them out while Chrome doesn't.
We use an opacity: 0.5 rule to grey them out ourselves in Chrome,
but in Firefox they are not dark enough to read easily because of
the double greying. I checked with the designers but they don't
have any better fix for that.

addons/web/static/src/css/base.css

@@ -2793,6 +2793,9 @@
   filter: alpha(opacity=50);
   opacity: 0.5;
 }
+.openerp .oe_list_content > tbody > tr > td.oe_list_field_boolean input:-moz-read-only {
+  opacity: 1;
+}
 .openerp .oe_list_content > tbody > tr:nth-child(odd) {
   background-color: #f0f0fa;
   background-color: #efeff8;