Add GitHub schema to Postgres

docker/supabase/volumes/db/init/05-github-schema.sql

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+CREATE EXTENSION IF NOT EXISTS citext WITH SCHEMA extensions;
+
+CREATE TABLE IF NOT EXISTS github_users (
+  id serial PRIMARY KEY,
+  login citext NOT NULL, -- the user name of the user, known as the `login` field in GitHub API responses
+  type citext check ( type IN ('user', 'organization') ) NOT NULL
+);
+
+CREATE TABLE IF NOT EXISTS github_repositories (
+  id serial PRIMARY KEY,
+  name text NOT NULL,
+  owner integer NOT NULL references github_users ON DELETE CASCADE,
+  UNIQUE (owner, name) -- a user cannot have the same repository, but a repository name can be repeated for many users (e.g. forks)
+);
+
+-- In the GitHub API, pull requests are considered issues, too.
+-- Therefore, we would group both pull requests and issues in a single
+-- table. For more information, read:
+-- https://docs.github.com/en/rest/reference/issues and
+-- https://docs.github.com/en/rest/reference/pulls
+CREATE TABLE IF NOT EXISTS github_issues (
+  id serial PRIMARY KEY,
+  number integer NOT NULL,
+  repo integer NOT NULL references github_repositories ON DELETE CASCADE,
+  type citext check ( type IN ('issue', 'pull_request') ) NOT NULL,
+  UNIQUE (repo, number) -- an issue number is unique across the whole repo, but several repos can have the same number
+);
+
+/* 
+  TODO:
+    * Add policies to restrict the users who can write into these
+      tables (preferably, only the search or parser microservice
+      should write).
+    * Add bridge tables that connect this GitHub information to
+      a posts table that contains the posts themselves.
+ */