Show release notes before auto-upgrading to a new version

In current version, when users are subscribed to "auto-beta channel updates", upgrades are silent.

Desired behavior :

• As a beta subscribed user, when the update is successful, i can see a new window with the title "Drive has been updated to version x.x.x beta" and display the release note from Jenkins/github
  > with user actions : acknowledge ("ok" button) and close window (both having the same behavior).