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

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

Lillith13 / Airburbs