Improve moved dir/file synchronization

Improvements for all users:

• Electron was upgraded from 1.7.9 to 1.8.3 (including quite a few security and
  bug fixes).
• The file/directory move synchronization logic (the part that takes
  already-merged metadata changes and turn them into actions on the other side)
  was substantially rewritten to make it more robust and handle more complex
  cases (e.g. moving a/ to b/ then c/ to a/, or moving a/ to b/ then
  deleting b/).
• A file won't be updated when only its mtime changed. This is a first necessary
  step to later stop computing checksums for files with unmodified mtime, making
  the initial scan faster.

Improvements for macOS and BSD users:

• We fixed a weird recurring error that was generating useless logging on every
  run.

Improvements for developers and support team:

• ES6 modules were replaced with node.js ones. This is a first necessary step
  to get rid of Babel, simplify our build process and make stacktraces smaller
  and easier to read.
• The app now waits for the configuration to be loaded before logging full
  client information (not just some part of it).
• We introduced a few jq filters to help
  filtering huge logs. Those should work well with multiple huge log files
  (e.g. > 1GB).

See also known issues.

Happy syncing!