[IMP]added subtype in blog to send notification of blog follower and add...

... follower in post when they comment on post.

Update website_blog.blog_post.js

Update test_ui.py

- Task: https://www.odoo.com/web#id=10039&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.cQKOHzo37oZPaAZS
Here the test is not complete I think. Here is the test I would write
1/ subscribe a test_user to a blog (done)
2/ create a new post, test_user is not in the followers
3/ publish the post, test_user is notified
4/ test_user comments the blog, he is added in the followers
5/ another_user comments the blog, test_user is notified