✨ Implement newsletter subscription form using API

Switch from SendInBlue to EmailOctopus for newsletter. Now uses their API through the form to handle subscription.

src/style/molecules/_newsletter-form.scss

@@ -1,5 +1,7 @@
 // Email Form
 .newsletter-form {
+    position: relative;
+
     &__email {
         position: relative;
         width: 100%;
@@ -23,7 +25,7 @@
             font-size: rem(16px);
             font-family: $font-sans;
             font-weight: 300;
-            color: #333;
+            color: $color-tertiary;
             background: none;
             border: none;
 
@@ -99,4 +101,24 @@
             }
         }
     }
+
+    // Message
+    &__message {
+        padding: 24px;
+        text-align: center;
+        border-radius: 8px;
+
+        &.is-error {
+            position: absolute;
+            top: 0;
+            left: 0;
+            width: 100%;
+            color: #fff;
+            background: $color-secondary;
+        }
+        &.is-success {
+            color: $color-text;
+            background: $color-tertiary;
+        }
+    }
 }