Make server endpoints more RESTful

In response to this piece of review left by jobara on the pull request for SJRK-289, the server endpoints for getting, saving and editing stories should be adjusted to be more RESTful and internally consistent with respect to the routes for stories and files. This work will also require updating the client side to call the correct endpoints.

jobara's original comments from October 5, 2020:

Should review the end points for proper RESTful semantics. In this case POST to create a story makes sense. However uploading a file would likely be better to POST to /stories/:id/files. Updating specific files, or stories can use PUT and deleting a file or story can use DELETE.