Consider mounting schedulers from `standalone` instead of `handler`.

While reviewing #2728 we noticed that the email (and now application eraser) poller is started in the `rems.handler` namespace with a require. Perhaps it would be nice to start them in the `standalone.clj` and possibly in some of the test fixtures.

The require is rather subtle, because the namespace is only required, but nothing from there is ever referenced. The comment in that line of code "to enable email polling" really seems like a stopgap measure and not a robust way to do this.

See also:
https://github.com/CSCfi/rems/issues/2102