504 Gateway Timeout on /org/example/servers/example/routes with large number of routes

## Describe the bug

On a setup where postal is deployed with docker-compose and is behind a nginx reverse proxy we get a 504 Gateway Timeout when clicking on the "Routing" link.

When we say we have a large number of routes, on this installation we count 20'000 routes. Server side we see the postal thread eating up a lot of CPU.

## To Reproduce

1. Login to https://postal-example/
2. Go to https://postal-example/org/example/servers/example/routes
3. Get a 504 Gateway Timeout

## Expected behaviour

A page showing the results or partial results. Maybe pagination would be appropriate in this case.

## Screenshots


## Environment details

- OS: server side debian + docker-compose
- Browser: n/a [e.g. chrome, safari]
- Version 2.1.2
- Type : desktop

## Additional information/context

A way to bypass this is to bypass the nginx, in our case we use `ssh -L 5000:localhost:5000` and connect using http://localhost:5000 the page is *very* long to compute and display but at least we don't get a 504.