Add SESSION_STORE options (#5403)

* Allow configuring the session storage driver

Fixes #3814

* Document SESSION_STORE env var

* Add missing dependency

* Docs tweak

api/src/middleware/session.ts

@@ -0,0 +1,20 @@
+import expressSession, { Store } from 'express-session';
+import env from '../env';
+import { getConfigFromEnv } from '../utils/get-config-from-env';
+
+let store: Store | undefined = undefined;
+
+if (env.SESSION_STORE === 'redis') {
+	const Redis = require('ioredis');
+	const RedisStore = require('connect-redis')(expressSession);
+
+	const redisClient = new Redis(env.SESSION_REDIS || getConfigFromEnv('SESSION_REDIS_'));
+	store = new RedisStore({ client: redisClient });
+}
+
+if (env.SESSION_STORE === 'memcache') {
+	const MemcachedStore = require('connect-memcached')(expressSession);
+	store = new MemcachedStore(getConfigFromEnv('SESSION_MEMCACHE_'));
+}
+
+export const session = expressSession({ store, secret: env.SECRET as string, saveUninitialized: false, resave: false });