Add unread indicator to conversations (#9009)

app/controllers/api/v1/conversations_controller.rb

app/controllers/api/v1/reports_controller.rb

app/javascript/mastodon/actions/conversations.js

...todon/features/direct_timeline/components/conversation.js

...ures/direct_timeline/containers/conversation_container.js

app/javascript/mastodon/reducers/conversations.js

app/javascript/styles/mastodon/components.scss

app/models/account_conversation.rb

app/serializers/rest/conversation_serializer.rb

config/initializers/doorkeeper.rb

config/routes.rb

...ate/20181018205649_add_unread_to_account_conversations.rb

db/schema.rb