[FIX] web: force red color for invalid form elements

When an input is saved with an invalid value, it gets the
`o_form_invalid` class. This makes the border red. In enterprise, there
was a bug as required inputs have also a different border color and the
rule was more important. Unfortunately there is no avoiding
the !important rule here. Indeed, other solutions would require to make
more rules in enterprise or to change the order of files (which would
create many problems).

addons/web/static/src/less/form_view.less

@@ -174,7 +174,7 @@
 
     .o_form_invalid {
         &.o_form_input, .o_form_input, &.o_form_textarea, .o_form_textarea {
-            border-color: @brand-danger;
+            border-color: @brand-danger!important; // enterprise o_form_required rule overrides this without !important
         }
     }