✨ Rework buttons classes and variants

Add a conditional class and list props as BEM class modifiers

src/components/organisms/Locations.svelte

@@ -47,7 +47,7 @@
         {#each continents as { name, slug }}
             <li class:is-disabled={currentContinent && currentContinent !== slug}>
                 <Button
-                    tag="button" text={name} class="button--small"
+                    tag="button" text={name} size="small"
                     on:click={() => filterLocation(slug)}
                 />
             </li>

src/components/organisms/Shop.svelte

@@ -29,11 +29,7 @@
             <h3 class="title-medium">{shop.module_title}</h3>
             <p class="text-small">{shop.module_text}</p>
             {#if shop.enabled}
-                <Button
-                    url="/shop"
-                    text="Shop"
-                    class="button--pink"
-                />
+                <Button url="/shop" text="Shop" color="pink" />
             {/if}
             <p class="detail">
                 Posters available for {locationsWithPoster.join(', ').replace(/,(?!.*,)/gmi, ' and')}.