So far, BridgeDB looked at the user's Accept-Language request header to
decide what language to use in its web interface.  Not everybody likes
that, so we should provide an option to override this behaviour.  This
patch adds a language switcher to BridgeDB's web interface.  It sits at
the top right and lets the user choose their language.

Some implementation considerations:

* The patch uses BridgeDB's "lang" HTTP GET argument to pass the chosen
  language from one page to another.  This allows us to avoid cookies.

* We allow the user to pick any language that BridgeDB supports,
  regardless of how complete the translations are.

* Each language in the language switcher is translated to the respective
  language, i.e., it says "español" instead of "spanish".

This patch fixes <https://bugs.torproject.org/26543>.