G-Cloud: add monthly graph

Add monthly as well as cumulative procurement graph on the G-Cloud dashboard: add new Backdrop stubs, add and update Stagecraft stubs. 

Also: 

* Change "Iaas" to "IaaS" in our stub data, to match Backdrop. 
* Ensure wording is consistent across graphs. 
* Update page-per-thing URLs to match `h1` text, as per our new convention.