Adds sentiment analysis to posts.

[mattrltrent]

Primary changes

• Adds the GET /posts/{post_id}/sentiment route. This allows for users to determine the sentiment of a post via passing in its masked ID.

Secondary changes

• Adds openapi.yaml documentation for the GET /posts/{post_id}/sentiment route.

Note

• This route will likely be trivial in nature. It exists for the sake of adding context to the app. Most users probably won't use it, but it gives them something else to explore, which I think is important.

Closes #35.