Refactoring

* Make permalinks nicer (removed dates)
* Remove useless 'includes', layouts are fine
* Rename blog posts to fit a unique URL template
* Fix CSS
* Add new categories (each talk now has 2 categories, one for the APIHour event, the other to mark it as a talk)
* Probably a few more fixes
* Use of 'post_url' when it makes sense
Add missing talks (Thomas, Pierre, and Olivier)