[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
Can you add a comment that explains what your are stripping here from the blog posts (links? attributes? tags?) and name your regexp variables accordingly.