state/apiserver: Return filename from Backup API

This change adds a Content-Disposition header to backup API responses
which contains the server-generated backup filename. This becomes
important once backups start being stored server side (soon) but also
simplifies the client side of the backup API (no need to generate
backup filenames).
Merge pull request #266 from mjs/backup-api-filename

state/apiserver: Return filename from Backup API

This change adds a Content-Disposition header to backup API responses which contains the server-generated backup filename. This becomes important once backups start being stored server side (soon) but also simplifies the client side of the backup API (no need to generate backup filenames).