Add support for custom emoji in reactions.

This seems to be working correctly, but element-desktop isn't rendering them,
not sure if that's expected or not.

Closes #4

database/migrations/03-emoji.sql

@@ -0,0 +1,5 @@
+CREATE TABLE emoji (
+	discord_id TEXT NOT NULL PRIMARY KEY,
+	discord_name TEXT,
+	matrix_url TEXT
+);

database/migrations/migrations.go

@@ -41,6 +41,7 @@ func Run(db *sql.DB, baseLog log.Logger) error {
 		migrator.Migrations(
 			migrationFromFile("01-initial.sql"),
 			migrationFromFile("02-attachments.sql"),
+			migrationFromFile("03-emoji.sql"),
 		),
 	)
 	if err != nil {