Update all code paths that pass file content to the storage layer to
provide io.ReadSeeker instead of io.Reader:
- Avatar upload: use bytes.NewReader instead of bytes.Buffer
- Background upload handler: use bytes.NewReader instead of bytes.Buffer
- Unsplash background: buffer response body into bytes.NewReader
- Dump restore: buffer zip entry into bytes.NewReader
- Migration structure: pass bytes.NewReader directly instead of wrapping
in io.NopCloser
- Task attachment: change NewAttachment parameter from io.ReadCloser to
io.ReadSeeker