Add administrator wishlist settings

Allows the administrator to implement the following settings concerning any wishlist in general:

• Default title of a wishlist.

• Permit a saved shipping address in a wishlist.

• Make users wishlist private by default.

• Allow users to make their wish list private.

• Show all wish lists by default.

• Allow users to add out of stock item into wish list.

• Default From address.

Implementation steps involved:

1. The feature makes use of the Form API for the creation and implementation of the above functionalities. The Form function has to be refactored to the FormStateInterface function.
2. The Configuration API provides a central location to store configuration details of the above components, particularly Drupal::config()→get() & Drupal::config()→set(), to store the involved variables.
3. The global theme variables have to be replaced by the active theme object.