Add 2015 dates, update sessions and booking link

* New dates for 2015
* Increasing take-up and assisted digital now covered in a combined day, Supporting people to use your digital service
* Design principles, Always be testing and Data-driven services in a combined day, Design and improve your digital service
* Procure to improve renamed to also include spend control and governance
* Procurement, agile and identity in a combined day, Practice, procurement and platforms for your digital service
* Request a place now by GDS Skills Team invitation only