Fixes #17866: Vary: Accept-Language header when language prefix used

Tests pass on 2.7 and 3.3.

Remarks:
- we cannot restrict including "Accept-Language" to only the case when it was used in page rendering. The language might be taken from the session or a client cookie. In that case the user gets that specific language but it doesn't mean the server doesn't vary content depending on Accept-Language (e.g. if the session would be empty and the cookie not set). Such a thing would break caching proxies. Taking that into account, I leave out "Vary: Accept-Language" on `i18n_patterns` URLs only.
- the patch reuses supported languages across requests to speed up computation, `trans_null` had to learn the `supported`` argument to ``get_language_from_path()`` (already in `trans_real`).
- now `LocaleMiddleware.is_language_prefix_pattern_used()` would be computed in every response so I moved this computation to `__init__` since it's not going to change between restarts
- on a similar note, `django.utils.translation.trans_real.check_for_language` is needlessly computed for every `get_language_from_path()`. It won't change between restarts of the application, shouldn't we memoize it?