Support legacy Confluence Storage format

[osnard]

There is the element <bodyType> within <object class="bodyContent"> elements

• Value of 0 means “legacy Confluence wikitext” (https://support.atlassian.com/confluence-cloud/docs/insert-confluence-wiki-markup/) → unconvertable
• Value of 2 means “modern HTML content” → covered by regular functionality

Good news is, that it apparently never caused an issue, as only very old revisions are affected and those are usually not part of the migration anyways.

Yet it makes sense to handle this case: #114