Lowercase "performance platform" for consistency

I don't have a strong opinion about whether it should be Performance
Platform or performance platform, but doing different things everywhere
makes me sad.
We're government, not Government
I used ack to get rid of All The Capitals
Very rough copyedit of user research surveys
Uppercase Performance Platform

...according to @sarahrichards:

https://github.com/alphagov/government-service-design-manual/pull/220#commitcomment-4250328
Minor typos and tweaks
Merge pull request #220 from alphagov/consistency-is-key

Style guide and performance platform case