Add Discord outbound posting (#14)

Stacks on #13. Adds discord_post_links and a subscriber that posts roots into a public thread, replies into that thread, and edits the linked Discord message. Unset Discord env leaves the site unchanged.

Reviewed-on: #14
Co-authored-by: codegirl-007 <s.raide@gmail.com>

db/queries/discord_links.sql

@@ -0,0 +1,32 @@
+-- name: GetDiscordPostLinkByPostID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE post_id = sqlc.arg(post_id);
+
+-- name: GetDiscordPostLinkByMessageID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_message_id = sqlc.arg(discord_message_id);
+
+-- name: GetDiscordPostLinkByThreadID :one
+SELECT post_id, discord_message_id, discord_thread_id, created_at
+FROM discord_post_links
+WHERE discord_thread_id = sqlc.arg(discord_thread_id)
+  AND discord_thread_id <> '';
+
+-- name: UpsertDiscordPostLink :exec
+INSERT INTO discord_post_links (
+  post_id, discord_message_id, discord_thread_id, created_at
+)
+VALUES (
+  sqlc.arg(post_id),
+  sqlc.arg(discord_message_id),
+  sqlc.arg(discord_thread_id),
+  sqlc.arg(created_at)
+)
+ON CONFLICT (post_id) DO UPDATE SET
+  discord_message_id = EXCLUDED.discord_message_id,
+  discord_thread_id = CASE
+    WHEN EXCLUDED.discord_thread_id <> '' THEN EXCLUDED.discord_thread_id
+    ELSE discord_post_links.discord_thread_id
+  END;