Remove arkency-command_bus dependency

[TastyPi]

AFAICT this is not referenced anywhere in the gem, and we (currently) don't plan to use it, so it would be nice to not need to install it.

[jandudulski]

This can come with a surprise for people that rely on the gem and do not specify it explicitly in the Gemfile. At least docs should also be updated that the gem has to be added manually.

[mostlyobvious]

To give some context first:

• on one hand the idea behind rails_event_store gem was to ship the event store (which in fact is implemented in the ruby_event_store) tailored to the Rails framework ecosystem (integration with ActiveJob, ActiveRecord, AcitveSupport::Notifications, generators, migrations, etc.)

• on the other hand it was an umbrella to include indirect dependencies — aggregate_root, and arkency-command_bus for quick and painless start (batteries included), in the direction of the framework on its own rather than just a minimal integration

The framework did not materalize so far. Both gems, the aggregate_root and arkency-command_bus could be dropped as mandatory dependencies. Doing so requires a deprecation notice first and major release second — some people already depend on those and we do follow semantic versioning to avoid unnecessary surprises.

What we could do to make the transition easier — is to include the gems in the APP_TEMPLATE along rails_event_store line. This would give batteries included start and a clear way to eject extra dependencies if not needed.