Make link styles in grid of co-op images more specific

So that they don't target other links, such as buttons, that appear
inside the grid.

assets/css/app.css

@@ -240,7 +240,7 @@ section, footer, header {
   align-items: stretch;
 }
 
-.grid a {
+.grid__link {
   display: flex;
   height: 100%;
 }

index.html

@@ -93,7 +93,7 @@ layout: default
 
   <div class="grid grid-dense">
     {% for coop in site.coops %}
-    <a href="{{ coop.url | relative_url }}">
+    <a class="grid__link" href="{{ coop.url | relative_url }}">
       <div class="box">
         <img class="greyscale" src="/images/coops/{{ coop.logo }}" alt="{{ coop.name }}" />
       </div>