'related' fails with error because it can't find 'mktime'

'time' is actually imported as 'time_module' within front.py. Updating the code to reflect this fixes the issue. 

```
File '/home/reddit/reddit/r2/r2/lib/validator/validator.py', line 191 in newfn
  return fn(self, *a, **kw)
File '/home/reddit/reddit/r2/r2/controllers/reddit_base.py', line 589 in new_fn
  return fn(self, **kw)
File '/home/reddit/reddit/r2/r2/lib/validator/validator.py', line 191 in newfn
  return fn(self, *a, **kw)
File '/home/reddit/reddit/r2/r2/controllers/front.py', line 743 in GET_related
  start = int(time.mktime((article._date - rel_range).utctimetuple()))
AttributeError: 'builtin_function_or_method' object has no attribute 'mktime'
```