Migrate Rspec feature tests to Cucumber

[zspencer]

We could probably do this in a series of small steps that are safe to merge independently:

1. Write the features/accepting_messages.feature file with scenarios inferred from the hosted/spec/features/guest_messages_inbox_spec.rb file
2. Fill in the step definitions and demonstrate the feature continues to work.
3. Delete the old hosted/spec/features folder

Reference Material:

• I spiked a bit on this for CompostPile: https://github.com/zinc-technology/compost-pile/tree/development/features
• Cucumber JS Repo: https://github.com/cucumber/cucumber-js
• Cucumber Docs: https://cucumber.io/docs/cucumber/