Remove CHAT_MIN_RANGE

...or at least make it optional.

According to main.js:
```js
// chat messages are requested for the visible viewport. On high zoom
// levels this gets pretty pointless, so request messages in at least a
// X km radius.
window.CHAT_MIN_RANGE = 6;
```

Actually, it isn't completely pointless. Sometimes I zoom very close to a specific portal because I only want logs for this portal. I've heard similar reports from other people as well. It happen to me that I wondered why the log contained entries for portals far away, as I didn't think of this range.

I want to discuss this here before making sudden changes.