Introduce abstraction for file storage

[odrotbohm]

A common theme in project topics is uploading files to the application and serving them back. This causes students to often struggle with technical details of finding a proper location to upload files to, and serve them via URIs in turn.

We should introduce an API that simplifies this with the following traits

• A default location of ~/.salespoint/uploads, configurable via an application property salespoint.storage.location.
• The ability to store files in user specific sub-buckets to prevent different users from accidentally overwriting content with the same file-name.