[Web] Increase polling frequency

At a recent meeting, there were incidences of simultanous checkouts
within the poll interval--i.e. mentor A checked out a student right
after mentor B checked out the same student, before mentor A's device
had polled for the updated information. This causes desync, and a 422
http error. The temporary solution is to poll more frequently, so that
the issue is less likely.

We should also probably handle the 422 errors gracefully, but if I
decide to do that, that'll happen in a separate commit.

attendance-web/src/environments/environment.prod.ts

@@ -2,5 +2,5 @@ export const environment = {
   production: true,
   apiRoot: "/attendance/api",
   authRoot: "/attendance/auth",
-  pollInterval: 5000
+  pollInterval: 500
 };

attendance-web/src/environments/environment.ts

@@ -6,7 +6,7 @@ export const environment = {
   production: false,
   apiRoot: "/api",
   authRoot: "/auth",
-  pollInterval: 5000
+  pollInterval: 500
 };
 
 /*