[MIG] website_blog: new api migration

- Task: https://www.odoo.com/web?debug=#id=9783&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.gSXNQBNaLZL5Ausm
I see your point. Ok then.

Oh, I propose to rename it to _postprocess_content, with two 's' :) . Add a backward compatility statement (_postproces_content = _postprocess_content).