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

### TODO
- [x] Apply first round of suggestion from the review
- [x] Refactor the list blog action
- [x] Make sure there is enough comments for new comers to understand the code.
- [ ] Create a dedicated (sub) Jira issue for the blog and blog_post