Improve rails generators

[paranoicsan]

Add/update generators to perform the following tasks:

• Generate .env files from a template
• Do not add https://rails-assets.org source from the very beginning as bundle is trying to fetch them initially. Need to add a note to add those sources by hand
• Add the following to application.rb for webpacker support:

config.middleware.use(
  Rack::Static,
  urls: ['/lcms_engine_packs'], root: File.join(Gem.loaded_specs['lcms-engine'].full_gem_path, 'public')
)