lib/model, lib/versioner: Prevent symlink attack via versioning (fixes #4286)

Prior to this, the following is possible:

- Create a symlink "foo -> /somewhere", it gets synced
- Delete "foo", it gets versioned
- Create "foo/bar", it gets synced
- Delete "foo/bar", it gets versioned in "/somewhere/bar"

With this change, versioners should never version symlinks.

lib/versioner/simple.go

@@ -26,6 +26,8 @@ type Simple struct {
 }
 
 func NewSimple(folderID, folderPath string, params map[string]string) Versioner {
+	cleanSymlinks(folderPath)
+
 	keep, err := strconv.Atoi(params["keep"])
 	if err != nil {
 		keep = 5 // A reasonable default
@@ -50,6 +52,9 @@ func (v Simple) Archive(filePath string) error {
 	} else if err != nil {
 		return err
 	}
+	if fileInfo.Mode()&os.ModeSymlink != 0 {
+		panic("bug: attempting to version a symlink")
+	}
 
 	versionsDir := filepath.Join(v.folderPath, ".stversions")
 	_, err = os.Stat(versionsDir)