Refactor login/logout mutations a smidge

I decided that `setAuthToken` was the more appropriate server-level API, and that letting the login/logout mutations engage with the auth library made more sense to me!

src/server/index.js

@@ -4,10 +4,7 @@ import { getUserIdFromToken as getUserIdFromTokenViaAuth0 } from "./auth";
 import connectToDb from "./db";
 import buildLoaders from "./loaders";
 import { plugin as cacheControlPluginFork } from "./lib/apollo-cache-control-fork";
-import {
-  getAuthToken,
-  getUserIdFromToken as getUserIdFromTokenViaDb,
-} from "./auth-by-db";
+import { getUserIdFromToken as getUserIdFromTokenViaDb } from "./auth-by-db";
 
 const rootTypeDefs = gql`
   enum CacheScope {
@@ -86,11 +83,11 @@ const config = {
     return {
       db,
       currentUserId,
-      login: async (params) => {
-        const authToken = await getAuthToken(params, db);
-        if (authToken == null) {
-          return null;
-        }
+      setAuthToken: (authToken) => {
+        if (authToken != null) {
+          // Set the auth token as a secure cookie, encoded as JSON! (We also
+          // url-encode it, which is pretty standard for cookie-writing - to
+          // the extent that `req.cookies` actually decodes it automatically.)
           const oneWeekFromNow = new Date();
           oneWeekFromNow.setDate(oneWeekFromNow.getDate() + 7);
           res.setHeader(
@@ -98,13 +95,10 @@ const config = {
             `DTIAuthToken=${encodeURIComponent(JSON.stringify(authToken))}; ` +
               `Max-Age=${60 * 60 * 24 * 7}; Secure; HttpOnly; SameSite=Strict`
           );
-        return authToken;
-      },
-      logout: async () => {
-        // NOTE: This function isn't actually async in practice, but we mark it
-        //       as such for consistency with `login`!
+        } else {
           // Set a header to delete the cookie. (That is, empty and expired.)
           res.setHeader("Set-Cookie", `DTIAuthToken=; Max-Age=-1`);
+        }
       },
       ...buildLoaders(db),
     };

src/server/types/User.js

@@ -1,6 +1,7 @@
 import { gql } from "apollo-server";
 
 import { assertSupportSecretOrThrow } from "./MutationsForSupport";
+import { getAuthToken } from "../auth-by-db";
 
 const typeDefs = gql`
   type User {
@@ -365,15 +366,16 @@ const resolvers = {
   },
 
   Mutation: {
-    login: async (_, { username, password }, { login }) => {
-      const loginToken = await login({ username, password });
-      if (loginToken == null) {
+    login: async (_, { username, password }, { setAuthToken, db }) => {
+      const authToken = await getAuthToken({ username, password }, db);
+      if (authToken == null) {
         return null;
       }
-      return { id: loginToken.userId };
+      setAuthToken(authToken);
+      return { id: authToken.userId };
     },
-    logout: async (_, __, { currentUserId, logout }) => {
-      await logout();
+    logout: async (_, __, { currentUserId, setAuthToken }) => {
+      setAuthToken(null);
       if (currentUserId == null) {
         return null;
       }