Performance: Build LANGUAGE_URL_MAP once on startup, rather than lazily

Got [a stupid-simple profiler middleware](https://djangosnippets.org/snippets/727/) working, and found something simple at the top of the list. 

Here's the profiler output before:
```
         525450 function calls (513252 primitive calls) in 5.618 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     1334    0.311    0.000    0.534    0.000 settings.py:178(lazy_lang_url_map)
    57579    0.230    0.000    0.230    0.000 {method 'lower' of 'str' objects}
     5803    0.226    0.000    1.253    0.000 tokenizer.py:59(__iter__)
```

And, here's the profiler output after:
```
         463496 function calls (451409 primitive calls) in 4.857 seconds

   Ordered by: internal time

   ncalls  tottime  percall  cumtime  percall filename:lineno(function)
     5803    0.202    0.000    1.173    0.000 tokenizer.py:59(__iter__)
 6190/749    0.185    0.000    0.571    0.001 copy.py:145(deepcopy)
     6616    0.170    0.000    0.760    0.000 _base.py:107(__iter__)
```