fix(files): update all callers to provide seekable readers for S3 uploads

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

pkg/models/task_attachment.go

@@ -58,8 +58,7 @@ func (*TaskAttachment) TableName() string {
 }
 
 // NewAttachment creates a new task attachment
-// Note: I'm not sure if only accepting an io.ReadCloser and not an afero.File or os.File instead is a good way of doing things.
-func (ta *TaskAttachment) NewAttachment(s *xorm.Session, f io.ReadCloser, realname string, realsize uint64, a web.Auth) error {
+func (ta *TaskAttachment) NewAttachment(s *xorm.Session, f io.ReadSeeker, realname string, realsize uint64, a web.Auth) error {
 
 	// Store the file
 	file, err := files.Create(f, realname, realsize, a)