Pageloader: Use only parsoid and remove hardcoded enwiki host

* Removed the MediaWiki page loader because we need parsoid actually
* Make the page loader work with any source language
* Added a bit of error handling

Change-Id: I6a5add64bf057575f42e2bb9635b0f2a506973bc
Merge "Pageloader: Use only parsoid and remove hardcoded enwiki host"