Add a new "wiki" soup.

We plan to gradually move wiki pages across from our GitHub wiki.
Add layout for "wiki" pages heavily based on the blog layout.

I've copied `soups/wiki/wiki-layout.haml` almost completely from
`soups/blog/blog-layout.haml`. The differences are as follows:

* page_title
* body CSS class is `wiki` instead of `blog`
* logo links to home page instead of blog home page

For the moment I've changed `public/stylehseets/screen.less` so that pages
using the new wiki layout use the same CSS styles as the blog pages.

Note that some of the blog "layout" actually happens in
`soups/blog/blog-template.haml` e.g. the sidebar, the comments, etc. But I
decided that we don't want that for now.

There's lots of duplication and other nastiness in this, but hopefully I don't
think it's making things particularly worse than they already were and hopefully
it's good enough to get started adding some wiki pages.
Add "accountant" page based on our internal wiki page.

See https://github.com/freerange/business/wiki/Accountant
Add "company-year" page based on our internal wiki page.

See https://github.com/freerange/business/wiki/Company-Year
Add "how-to-submit-annual-return" page based on internal wiki.

See https://github.com/freerange/business/wiki/Submit-our-Annual-Return