message: API: Handle absolute ResponsiveImageService paths
  Plus minor refactoring to use path.join.
diff: import sharp = require('sharp');
  +import config from '../config/main';
  import * as fs from 'fs';
  +import * as path from 'path';
  import {BreakpointSize} from '../models/BreakpointSize';
  import {IResponsiveImageData} from '../../../shared/models/IResponsiveImageData';
  @@ -72,15 +74,15 @@ export default class ResponsiveImageService {
  }
  await resizeOptions
  - .toFile(directory + '/' + fileNameToSave);
  + .toFile(path.join(directory, fileNameToSave));
  await retinaResizeOptions
  - .toFile(directory + '/' + retinaFileNameToSave);
  + .toFile(path.join(directory, retinaFileNameToSave));
  -
  - breakpoint.pathToImage = directory + '/' + fileNameToSave;
  - breakpoint.pathToRetinaImage = directory + '/' + retinaFileNameToSave;
  + const directoryRelative = path.relative(path.dirname(config.uploadFolder), directory).replace(/\\\\?/g, '/');
  + breakpoint.pathToImage = path.join(directoryRelative, fileNameToSave);
  + breakpoint.pathToRetinaImage = path.join(directoryRelative, retinaFileNameToSave);
  }
  if (!keepOriginalFile) {
  