message: FormField: add padding to unveil border, inherit border radius for children
diff: .FormField {
  position: relative;
  + box-sizing: border-box;
  + padding: 1px;
  }
  .FormField__border {
  border-color: var(--icon_tertiary_alpha);
  }
  -.FormField--ios .FormField__border {
  +.FormField--ios {
  border-radius: 10px;
  }
  -.FormField--android .FormField__border {
  +.FormField--android {
  border-radius: 8px;
  }
  -.FormField--vkcom .FormField__border {
  +.FormField--vkcom {
  border-radius: 6px;
  }
  +.FormField > :first-child,
  +.FormField__border {
  + border-radius: inherit;
  +}
  +
  .FormItem--error .FormField__border {
  background: var(--field_error_background);
  border-color: var(--field_error_border);
  