Add seed backup introspection

/renter/backup and /renter/recoverbackup are now deprecated, and /renter/uploadedbackups has been deleted. In their place, we have three new routes:

• GET /renter/backups
• POST /renter/backups/create
• POST /renter/backups/restore

/renter/backups now lists the contracts that are storing all known backups. You can also query the endpoint with ?host=abcdef... to download the list of backups stored on a specific host.

Also, all backups are now remote backups. Local backups can only be created/loaded using the deprecated routes.