Apply brand guidelines to site

Creative Co-op have recently prepared some brand guidelines for
CoTech, including a new logo, typography suggestions and background
images. This commit restyles the site to apply, as best we can, these
guidelines. It's hard to split this into smaller commits because the
decision to use a black background colour means many things have to
change.

Notably we've removed the "about/join" footer, as it was hard to
choose a bg colour and without one it became more apparent that this
footer was more in the way than useful.

Co-authored-by: Natasha Natarajan <natasha@outlandish.com>

index.html

@@ -3,7 +3,7 @@ title: CoTech
 layout: home
 ---
 
-<section class="slice slice--light-grey">
+<section class="slice">
   <div class="metadata">
     <div class="metadata_item">
       <h6 class="metadata_heading">Co-ops</h6>
@@ -31,7 +31,7 @@ layout: home
   </div>
 </section>
 
-<section class="slice slice--light-grey">
+<section class="slice">
   <p>Co-tech is a network of ethical tech, digital and creative co-operatives.</p>
   <p>We're part of <a href="https://www.workers.coop">workers.coop</a>, <a href="/join">join us!</a></p>
   <p>Working with co-operatives has major benefits: you’ll work directly with the people who own the business, who have a stake in making sure your project succeeds. With no private shareholders drawing out dividends, you’ll get genuine value for money.  And on top of that, you’ll get the benefit of working with small and responsive businesses which draw on the skills, specialisms and security of a large network.</p>