Define Rails::Engine in the Agent GEMs created

[alessio-signorini]

While building an agent as a GEM I found some difficulties adding a custom UI due to the views paths scope.

This PR declares a Rails Engine inside the GEMs created by this module so they will be able to define additional models, views, controller and routes.

Once #2659 is merged in the main Huginn codebase, it will be sufficient to create a partial in app/views/agents/agent_views/<agent_name>/_show.html.erb to have it appear in Huginn exactly as if it was located in the main codebase.

[alessio-signorini]

Existing agents can take advantage of this just adding

class Engine < ::Rails::Engine; end

in their /lib/huginn_whatever_agent.rb file.

[dsander]

That looks like a very clean solution overall!

[knu]

Nice!