Added support for "sharedSlug" where multiple uploaders can share files

[fabawi]

Added an option to upload to a shared slug. If the shared slug is specified on upload, the short and long slugs are still generated specifically for that connection. Additionally, the shared slug will provide a connection for each uploader independently. This approach respects the current filepizza specifications, including multiple file uploads, and password setting for individual uploader files.

Currently, I do not implement means for deleting stale uploader connections since there are multiple ways to deal with it, and I would rather leave it to the filepizza authors to decide how to approach this.

Future Suggestion:

• ATM, each uploader is enumerated during download. Uploaders could potentially also specify a username during upload
• If one downloader cannot connect to an uploader, they could be deleted from the additionalUploaders list. This is problematic however, since one downloader being unable to establish a connection with an uploader does not mean the upload is not available for others
• Add a keep-alive signal that maintains connections to available uploaders. The upload connection is discarded after multiple failed attempts to connect
• Alternatively, uploaders could be discarded independently after a static TTL is exceeded