mediawiki.api.parse: Specify wikitext as the content model

The parse API defaults to use the content model of the title, which
defaults to "API" in the main namespace which can be configured to have
a different content model.

Bug: 48201
Change-Id: I51143873e3782df4df500194a7ac71c4b446746b
Merge "mediawiki.api.parse: Specify wikitext as the content model"