Proposal: remove discussions code

Created by: keegancsmith

Discussions is an unused product and has remained that way for a long time. I propose we remove the code from our codebase. I'm not sure of a reason to keep it, but I may be missing some context.

cc @sqs @christinaforney @slimsag

• remove from webapp https://github.com/sourcegraph/sourcegraph/pull/10262
• remove from GraphQL API https://github.com/sourcegraph/sourcegraph/pull/10255
• remove site config feature flag
• remove backend code
• backup sg.com discussions in postgres
• drop db tables