Improve Language Redirection

[pmac]

In bedrock we do redirection for URLs with no locale in them potentially 2 times.

1. When the user first hits such a URL (e.g. /firefox/)
2. Then potentially again if the best supported locale for the user doesn't have a translation for that page

An issue has arisen due to those two redirection and locale choosing mechanisms behaving differently. In bug 1743894 you can see an example.

The parsing and processing of the info in the Accept-Language header in the LocaleURLMiddleware class is handled mostly by code in the urlresolvers.py file. The 2nd redirect is handled by code in the l10n_utils module. The former seems to be doing a better job following the proper procedure outlined in the bug, while the latter is more naive. We should bring the 2nd redirect up to standard.