Styled the add-tag-button to keep consistency

This is to keep consistency between this bundle and eZ Platform's design

bundle/Resources/public/admin/css/field/tagssuggest.css

@@ -19,6 +19,28 @@
     transition: border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
 }
 
+.ng-tags-input-ui button.button-add-tag {
+    color: #fff;
+    background-color: #f15a10;
+    border-color: #f15a10;
+    display: inline-block;
+    font-weight: 400;
+    text-align: center;
+    white-space: nowrap;
+    vertical-align: middle;
+    user-select: none;
+    border: 1px solid transparent;
+    padding: 0.375rem 0.75rem;
+    font-size: 1rem;
+    line-height: 1.5;
+    border-radius: 0.25rem;
+    transition: color 0.15s ease-in-out, background-color 0.15s ease-in-out, border-color 0.15s ease-in-out, box-shadow 0.15s ease-in-out;
+}
+
+.ng-tags-input-ui button.button-add-tag.button-disabled {
+    background-color: rgba(241, 90, 16, 0.7);
+}
+
 .ng-tags-input-ui .tagssuggestfield:focus {
     outline: 0;
     box-shadow: 0 0 0 0.2rem rgba(241, 90, 16, 0.25);