Fix sites in grid not having all the same width

Due to [`1fr` = `minmax(auto, 1fr)`](https://stackoverflow.com/questions/52785750/prevent-grid-area-from-expanding-causing-whole-page-to-scroll).

Author: meduzen

src/sass/layout/_sites.scss

@@ -1,9 +1,10 @@
 .sites {
   display: grid;
+  grid-template-columns: minmax(0, 1fr);
   grid-gap: 1.5rem;
 
   @media (--tablet-7) {
-    grid-template-columns: 1fr 1fr;
+    grid-template-columns: repeat(2, minmax(0, 1fr));
     grid-gap: 2rem;
     align-items: stretch;
   }