CE-306: Testing Infrastructure + Engine Rebuild

[toastercup]

Purpose
To accomodate our new testing infrastructure, Cortex was migrated to an Engine, so that a parent application can compose it as a dependency along with any other plugins. An example parent application may be a dummy test app, or it may be the actual host Rails application for a company's Cortex instance, such as CareerBuilder.

JIRA
https://careerbuilder.atlassian.net/browse/CE-306

Changes

• Improvements and fixes

  • Refactors codebase into a Rails Engine
  • Webpacker-related assets now distributed as Yarn package
  • Prop up dummy spec app that mounts Cortex Engine
  • Initial RSpec config
  • Namespace all constants/assets within Cortex namespace
  • New material-icons utility classes
  • Migrate all client-side dependencies from Gems to Yarn packages
  • Switches to Sprockets v4 manifest for asset config
  • Cleans up gemspec/Gemfile; unnecessary dependencies removed
  • Expanded/cleaned up gitignore
  • Prep for Rails 5.2
  • Application + Devise layout improvements
  • Devise config + locale improvements
  • Mailer config + layout improvements
  • Binstub improvements
  • Removes Hashr dependency
  • Code refactors + cleanup

• Changes to developer setup/environment

  • Yes - please review PR

• Architectural changes

  • Yes - please review PR

• Migrations or Steps to Take on Production

  • This will require a full rebuild of all servers

• Library changes

  • Yes - cortex-plugins-core has been updated to be compatible with these changes.

• Side effects

  • N/A

Screenshots

• Before
  N/A

• After
  N/A

Feature Server
No servers are currently compatible with the Beta Cortex Rebuild. We'll have a story in an upcoming sprint to set these up. In the meantime, you'll want to test this out by cloning and launching https://github.com/cortex-cms/cortex-starter

How to Verify These Changes

• Specific pages to visit

  • Test whole application

• Steps to take

  • Test whole application

• Responsive considerations

  • N/A

Relevant PRs/Dependencies

• COR-306: Testing Infrastructure cortex-plugins-core#68
• https://github.com/cortex-cms/cortex-starter (new repo created)

Additional Information
N/A

[toastercup]

@MKwenhua @mrfantasticwonders this PR is ready for your review.