add dynamodb support

[ccbrown]

The tracker is going ✨serverless✨.

The goal is more effective cost and scalability.

[ccbrown]

For posterity, migrating from a DB file to a DynamoDB table can be done like so:

func migrate() {
	config, err := external.LoadDefaultAWSConfig()
	if err != nil {
		log.Fatal(err)
	}
	dest, err := server.NewDynamoDBDatabase(dynamodb.New(config), viper.GetString("dynamodb-table"))
	if err != nil {
		log.Fatal(err)
	}
	defer dest.Close()
	source, err := server.NewBoltDatabase(viper.GetString("db"))
	if err != nil {
		log.Fatal(err)
	}
	defer source.Close()

	for _, locale := range server.Locales {
		activity, _, err := source.Activity(locale, "", 1000000000000000000)
		if err != nil {
			log.Fatal(err)
		}
		if err := dest.AddActivity(activity); err != nil {
			log.Fatal(err)
		}
	}
}