Context

We used the mainBody attribute to transmit the references list item id of the list entry containing the html content. Now we're using the more sustainable mwBodyHtml to directly transmit that content in mwData similar to how it's done for simple reference tags and their content.

Cleanup

• Remove the creation of the mainBody when converting to Html for Parsoid in toDomElements - Parsoid does not consume this attribute anymore
• Look into removing the mainBody in Parsoid output and remove the consumption in toDataElement

I'm not to sure about the latter and if it would make sense to have it as a fallback. As alternative I would support renaming it do something like mainBody.id to match body.id for the simple reference content.