Move auto suggest URL from global settings to server config

Problem/Motivation

The current implementation puts the auto suggest core URL in global settings, limiting websites to one autocomplete for the entire site. As noted in the docs on the Search Stax side, auto suggest is a per-app feature, so the suggester URL should be with the index update endpoint URL and read/write token in the server config.

Steps to reproduce

Try to implement more than one autocomplete.

Proposed resolution

Move the suggester URL config from global settings to the server. Update the necessary PHP code to use the new location and ensure a site can have multiple (different) autocompletes.