Some clean-up and fixes for the external API

* Use relative redirects so that they work both using internal and external
  IPs / addresses
* Remove /en/ style default prefixes in favor of production /enwiki/ etc to
  avoid cache misses
* Fix an encoding issue this uncovered in parserTests. Did not investigate too
  deeply why the language change triggered this, but the bug needed fixing in
  any case.

Change-Id: Iccc1fcf2ded40fb07319092070a2704366880c9c