Add GitHub schema to Postgres

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

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+CREATE TABLE IF NOT EXISTS github_users (
+  id serial PRIMARY KEY,
+  login text, -- the user name of the user, known as the `login` field in GitHub API responses
+  type text check ( type IN ('user', 'organization') )
+);
+
+CREATE TABLE IF NOT EXISTS github_repositories (
+  id serial PRIMARY KEY,
+  repo_name text,
+  owner integer references github_users ON DELETE CASCADE
+);
+
+-- 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,
+  repo integer references github_repositories ON DELETE CASCADE,
+  type text check ( type IN ('issue', 'pull_request') )
+);
+
+/* 
+  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.
+ */