admin: Implement sychronization backends

We are now able, depending on configuration, to retrieve files from either WebDAV or local file system.

libfic/file.go

@@ -135,7 +135,23 @@ func (e Exercice) ImportFile(filePath string, origin string, digest []byte) (int
 			}
 		}
 
-		return e.AddFile(strings.TrimPrefix(filePath, FilesDir), origin, path.Base(filePath), result512, fi.Size())
+		dPath := strings.TrimPrefix(filePath, FilesDir)
+		if f, err := e.GetFileByPath(dPath); err != nil {
+			return e.AddFile(dPath, origin, path.Base(filePath), result512, fi.Size())
+		} else {
+			// Don't need to update Path and Name, because they are related to dPath
+
+			f.IdExercice = e.Id
+			f.origin = origin
+			f.Checksum = result512
+			f.Size = fi.Size()
+
+			if _, err := f.Update(); err != nil {
+				return nil, err
+			} else {
+				return f, nil
+			}
+		}
 	}
 }