More work on permission utils and eager permission checks

backend/src/utils/getMissingChannelPermissions.ts

@@ -0,0 +1,15 @@
+import { Constants, GuildChannel, Member, Permission } from "eris";
+import { getMissingPermissions } from "./getMissingPermissions";
+
+/**
+ * @param requiredPermissions Bitmask of required permissions
+ * @return Bitmask of missing permissions
+ */
+export function getMissingChannelPermissions(
+  member: Member,
+  channel: GuildChannel,
+  requiredPermissions: number | bigint,
+): bigint {
+  const memberChannelPermissions = channel.permissionsOf(member.id);
+  return getMissingPermissions(memberChannelPermissions, requiredPermissions);
+}