Disable phase date fields when a phase is disabled

The JavaScript involved wasn't working since we removed the disable-date
attribute in commit 73ff6881.

We're also improving the JavaScript in two ways:

First, we trigger the `change` event immediately, so when the page loads
date fields are disabled when phases are disabled.

And second, we don't remove the selected dates when disabling a phase,
so disabling it and enabling it again will keep the selected values.

app/assets/stylesheets/layout.scss

@@ -1074,6 +1074,10 @@ form {
     &.error {
       margin-bottom: rem-calc(1);
     }
+
+    &:disabled {
+      background-color: $input-background-disabled;
+    }
   }
 
   [type="checkbox"] + label,