authn: Explicitly assign a unique id to the Passport strategy

Passport implicitly defaults the id to each strategy's .name property,
but then requires explicitly specifying those same names later in calls
to passport.authenticate().  Make the link more obvious by assigning our
own explicit ids, especially given anticipation of additional strategies
for API authn.

authn.js

@@ -39,8 +39,16 @@ const COGNITO_CLIENT_ID = PRODUCTION
   ? "rki99ml8g2jb9sm1qcq9oi5n"    // prod client limited to nextstrain.org
   : "6q7cmj0ukti9d9kdkqi2dfvh7o"; // dev client limited to localhost and heroku dev instances
 
+/* Arbitrary ids for the various strategies for Passport.  Makes explicit the
+ * implicit defaults; uses constants instead of string literals for better
+ * grepping, linting, and less magic; would be an enum if JS had them (or we
+ * had TypeScript).
+ */
+const STRATEGY_OAUTH2 = "oauth2";
+
 function setup(app) {
   passport.use(
+    STRATEGY_OAUTH2,
     new OAuth2Strategy(
       {
         authorizationURL: `${COGNITO_BASE_URL}/oauth2/authorize`,
@@ -177,12 +185,12 @@ function setup(app) {
       }
       next();
     },
-    passport.authenticate("oauth2")
+    passport.authenticate(STRATEGY_OAUTH2)
   );
 
   // Verify IdP response on /logged-in
   app.route("/logged-in").get(
-    passport.authenticate("oauth2", { failureRedirect: "/login" }),
+    passport.authenticate(STRATEGY_OAUTH2, { failureRedirect: "/login" }),
     (req, res) => {
       // We can trust this value from the session because we are the only ones
       // in control of it.