Automatic update fixes

This is for testing purpose only.
Do not install it, unless you know what you're doing.

Improvements for all platforms:

• When an update was available, the app used to display the version number and
  the release notes in the Settings. This was broken while migrating to the new
  auto-update system. The issue is partially fixed, but it only shows the
  summary instead of the full release notes for now.
• The auto-update migration also revealed another issue: the app was checking
  for updates every 1 or 2 minutes, triggering successive downloads of the
  latest release. This should work as expected now.

Improvements for Windows users:

• The app was not downloading updates from the appropriate location on Windows
  (while it was working on other platforms). This was fixed too.

Happy syncing!