Only add WikiEditor for content model WikiText

Together with solving bug 29908, this makes the presence of the
toolbar consistent. It is added to pages handling wikitext.

Note that we provide the CodeEditor on all JS/CSS/Lua pages.

Bug: 24713
Bug: 24041
Change-Id: Iaa6b9a6c511380ed1669d8b95cba4a3ddc6f9ab6