EZP-21532: blog and blog_post using symfony stack

Link to the issue: https://jira.ez.no/browse/EZP-21532

### Description
We want to migrate the demo bundle to use the symfony stack as much as possible. I started by migrating the blog and blog_post pages.

There is some TODO left, because we do not have a keyword service (or API) yet. I still managed to have most of it working using legacy callbacks or modifying the expected result.

# Test 
Manuel tests