Site copy rewrite

This branch is the site copy update detailed in https://docs.google.com/a/mysociety.org/document/d/1PYHl6Qsq7KpjDwDv3GSUwaxC5g34qcTf1JfG5q3pG_Y/edit#

Still to do, before review:
- [ ] Update multilingual versions (are these visible yet?)
- [ ] Finish resources page copy
- [ ] Add a new page for 'use a component' (copy needs finishing)