Rename cod-btn into cod-button.

frontend/src/design_system/button.scss

@@ -1,4 +1,4 @@
-button.cod-btn, a.cod-btn {
+button.cod-button, a.cod-button {
     padding: .8em 1.2em;
     border-radius: 10em;
     border: none;
@@ -38,6 +38,6 @@ button.cod-btn, a.cod-btn {
     }
 }
 
-a.cod-btn {
+a.cod-button {
     text-decoration: none;
 }