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/database.go

@@ -22,6 +22,7 @@ type Database struct {
 	Message    *MessageQuery
 	Reaction   *ReactionQuery
 	Attachment *AttachmentQuery
+	Emoji      *EmojiQuery
 }
 
 func New(dbType, uri string, maxOpenConns, maxIdleConns int, baseLog log.Logger) (*Database, error) {
@@ -79,5 +80,10 @@ func New(dbType, uri string, maxOpenConns, maxIdleConns int, baseLog log.Logger)
 		log: db.log.Sub("Attachment"),
 	}
 
+	db.Emoji = &EmojiQuery{
+		db:  db,
+		log: db.log.Sub("Emoji"),
+	}
+
 	return db, nil
 }