User preferences, notifications and contributors

[madhums]

closes #44 #39 #94

Adds user preferences model with the following notification preferences:

Discussion

• New discussion on a story I created

Post

• New post on a discussion I created
• Anyone posts in a discussion I participate in

Story

• New story was added

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Make sure to run once deployed

rake db:migrate
rake preferences:create_missing_preferences

Todo:

• add user preferences for all the above (models, tests)
• add user preferences UI
• add notifications
  • for when new stories are created
  • when someone creates a new discussion on my story
  • when someone creates a new post on a discussion I created
  • when someone posts on a discussion I am participating in
• add tests for the above notifications
• add notifications web UI
  • add notifications read/unread
  • use turbo stream to deliver notifications on the fly
  • add visual indicator in the header about the new notifications
  • display count perhaps?
• add user preferences to navigation
• add notifications to navigation
• add email footer with a message saying the user can change their preferences in case they don't wish to receive emails
• add test to make sure preferences are created after confirmation or invitation acceptation