Components apply page

This branch contains all of the changes from https://github.com/ciudadanointeligente/home-poplus/pull/54 (the new, Get Involved and Components sub pages) plus a new page explaining how to apply to make a component.

Changes to [the page content](https://github.com/mysociety/home-poplus/blob/abbf1c514bce6a990c633b9c115c62234b4b4167/components/apply/index.md) are welcome. Otherwise, this is good to go.