Add an Image to a Spot based on the Spot's id

[AmusedPocket]

Create and return a new image for a spot specified by id.

• [x] An authenticated user is required for a successful response
• [x] Only the owner of the spot is authorized to add an image
• [x] New image exists in the database after request
• [x] Image data returned includes the id, url, and preview
• [x] Error response with status 404 is given when a spot does not exist with the provided id