Disabling a flag also dis-allowlists all

I actually think the previous approach makes more sense, but it
wasn't the behaviour that we intended - the allowlists were
intended to be a restriction on the `enabled: true` property (i.e.
clearing the allowlist enabled it for everyone), rather than a
loosening of `enabled: false`. This is also why we have the
`ignoreAllowlist` option, to allow the use of feature flags on
public pages where we do not have a specific user.

We might still want to consciously choose the previous behaviour
(yes please!), but then we should replace the `ignoreAllowlist`
option by a `disableAllowlist` option that only removes the
ability to expose the flag to specific users.

src/db/tables/featureFlags.ts

@@ -67,9 +67,12 @@ export async function getEnabledFeatureFlags(
     .and.where("is_enabled", true);
 
   if (!options.ignoreAllowlist) {
-    query = query.and
-      .whereRaw("ARRAY_LENGTH(allow_list, 1) IS NULL")
-      .orWhereRaw("? = ANY(allow_list)", options.email);
+    query = query.andWhere(
+      (whereBuilder) =>
+        void whereBuilder
+          .whereRaw("ARRAY_LENGTH(allow_list, 1) IS NULL")
+          .orWhereRaw("? = ANY(allow_list)", options.email),
+    );
   }
 
   const enabledFlagNames = await query;