Fix adding custom interwiki prefixes

Adds a ParsoidConfig object, and uses it for interwiki prefix handling.

Currently includes the interwiki information that used to be in env, but now
needs to be elsewhere due to the async-ness of environment constructors.

Also fixed some other minor bugs from the config patch.

Change-Id: I8b5dd5a562a74f6ca2fe5d9d8e0b334659ddbeaa
Merge "Fix adding custom interwiki prefixes"