FeatureRequest: Allow user-defined resolve method for runpy host ip

Currently, the `exechost` is resolved by [`socket.gethostbyaddrsocket`](https://github.com/inducer/relate/blob/master/course/page/code.py#L830) when making the feedback for code questions. When the exechost is not localhost, that require a few seconds for the code to run, if the exechost ip is not resolvable. That lower the speed for response.

```python

import socket
def get_host(i):
for ip in range(i):
try:
socket.gethostbyaddr("192.168.1.%s" % ip)
except socket.error:
pass

%timeit get_host(10)
```

The result is:
```
1min 21s ± 105 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
```

My idea is to provide user with `settings.RESOLVE_RUNPY_HOST` (a `bool`) to allow user to determine whether to resolve the ip. If not, return the ip. along with `settings.RUNPY_HOST_MAP` (a 'dict'), to map ip/hostnames to strings, for the purpose, for example, to mask the real ip address.