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>

internal/store/sqlc/models.go

@@ -9,6 +9,13 @@ import (
 	"time"
 )
 
+type DiscordPostLink struct {
+	PostID           string
+	DiscordMessageID string
+	DiscordThreadID  string
+	CreatedAt        string
+}
+
 type Post struct {
 	ID        string
 	ParentID  sql.NullString