Enhancements and fixes in some existing generators

[coolprobn]

Enhancements

• Update regex in gem_installed? helper method to also look at spaces before and after "gem" text. Also ignore if the gem is there but commented out in which case we need to install the gem.
• Update check_and_install_gem helper method to use gem_installed? helper method instead of rewriting similar set of code
• Update devise/doorkeeper generator to use check_and_install_gem so same gem isn't installed multiple times
• Add run_db_migrate in devise generator which defaults to false. The reason for this is, in Zero Config Rails Avo generator throws error because it looks at User attributes but migration wouldn't have run by that time to return correct attributes.
• Make dotenv generator idempotent
• Bring back reek and rubocop extensions for Pronto that was mistakenly removed from previous release
• Update VCR generator to only install extension gems if they aren't already installed