Content updates to reflect project progress

Closes #17. Tagging @RuthC in case she wants to review the new text.

@webal A big chunk of this page, from "Areas of Collaboration" downwards, is missing from the repo but still appears here: http://stage.blocked.org.uk/getinvolved.html. Do you have changes in your local copy of the repo that you haven't pushed back to origin?

(I suggest we use pull requests to discuss content changes, then we can merge and close them once everyone's happy with the text, and someone has transferred the new content to staging.)