Register footer menus as theme locations

[stduerre]

Summary

Register footer menus so they are available as positions in WordPress admin.

Currently, the menus in the footer (Footer Primary, Footer Secondary, Social) are only defined by naming convention. That doesn't work for multilingual setups because menu names are unique and menus that work with the master theme can not be created in more than one language at a time. Registering the menu as theme locations enabled the use of WordPress admin to select what menus to show in the footer, independent of menu names.

If no menu is set for the location, the old naming convention is still in use to ensure backwards compatibility.

Testing

1. Make sure existing menus are still shown in the footer after installing this update (backwards compatibility)
2. Create or edit menus and assign them to the appropriate theme locations
3. Make sure the menus display correctly