[blog] Implement RSS Feeds

- Task: https://www.odoo.com/web?debug#id=9547&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.SIeozbGyEfAhkzKO
Not sure this is correct. We want the language from the blogpost, not the language from the context (that will be the language of the user, often public)