✨ Rework buttons classes and variants

Add a conditional class and list props as BEM class modifiers

src/style/molecules/_newsletter-form.scss

@@ -37,19 +37,12 @@
             }
         }
         button {
-            display: flex;
-            justify-content: center;
-            align-items: center;
             position: absolute;
             right: 16px;
             top: 50%;
             transform: translateY(-50%);
             margin-left: 32px;
-            background-color: $color-secondary;
-            height: 32px;
-            width: 32px;
             flex-shrink: 0;
-            border-radius: 100vh;
         }
 
         // States