Allow multiple entity types to be selected when creating an “Index”

Problem/Motivation

Allow multiple entity types to be selected when creating an “Index”. Currently we can only create a "node" index and a "user" index but there is no easy - out of the box - way to get this content together in one search page. It requires a contributed module (search_api_multisite I think) to combine those and it's not very well supported.

This issue is to really make that a core possibility in Search API D8.

Proposed resolution

Embed the search api multisite module in Search API core and fully support it.

Remaining tasks

Unidentified

User interface changes

Unidentified

API changes

Unidentified