Allows a user to save content to view later.

[mattrltrent]

Primary changes

• Adds POST /users/saved/ that allows users to save a comment or post to view later.
• Adds DELETE /users/saved/ that allows users to delete some bit of content they've saved.
• Adds GET /users/saved/ that allows users to paginate through all their saved content.

Secondary changes

• Adds documentation in openapi.yaml for routes: POST DELETE GET /users/saved/.

Notes

• This adds the ability to save posts, and includes the template for saving comments. It won't work properly for comments yet because we don't have comments implemented at all in the app. Does this mean this PR shouldn't be merged until comments are added?

Closes #11.