Partitioning of screenshot dir for large projects

[pbvahlst]

Partitioning of screenshot dir. Only if necessary.

Will only work if id's are used instead of url-filenames.

• Use each letter in the id-alphabet as name for a folder (36 letters).
• calculate the depth e.g.

    calculateDirPartitionDepth(maxFilesPrDir, totalFiles, alphabetSize) {
      let capacity = maxFilePrDir;
      let partitionDepth = 0;
      while (capacity < totalFiles) {
         capacity *= alphabetSize;
        partitionDepth++;
      }

      return partitionDepth;
    }

• Create the dirs ad hoc (not in advance)
• Make a config.json which saves the partitionDepth in the project path, so we
  can use this for resumes (passing in a folder with existing data)