Move search to SOLR and drop support for searching against the database

This issue outlines a plan for 13.0.0 of Open Social.

Problem/Motivation

Open Social provides a complex community product. Search is a powerful way for users to find content within their community and ensuring that search results are accurate and provided in a performant manner is important to a successful community.

The search_api module provides a way to easily abstract over different search back-ends. However to support those different back-ends Open Social is limited to only using functionality that's within the least common denominator. This can have an adverse effect on the user experience.

In our community building experience we've seen first-hand that using the database as a search back-end can have a profoundly negative impact on platform performance. Additionally we've been unable to build some desired features which would've been supported by SOLR but were not possible with a database.

It's with this in mind that for all of our SaaS customers we've already configured a SOLR back-end and do not use the database for search anymore. Unfortunately with the work being done for improved profile configuration and privacy options we're finding that the maintenance cost for making things work with the database back-end is becoming too high.

Proposed resolution

We plan to drop support for the search_api_db back-end 13.0.0. The social_search module's dependency will be changed from search_api_db to search_api_solr and an update hook will be provided to introduce the social_solr server, switch the indices and disable the search_api_db module.

Remaining tasks

User interface changes

API changes

Data model changes