Manage newreleases.io resources

[nicmr]

• implements the following subcommands:
  • newreleases diff
  • newreleases diff diverged
  • newreleases import
  • newreleases apply
• the following suggestions from the last planning meeting have been introduced:
  • email settings have global flag, with optional local override
  • URL field has been removed to reduce config bloat
• In addition, the following changes have been made:
  • remove all managing of integration with third party services. These can't be updated by the API anyway (unfortunately) so it makes no sense to store their configuration locally.
• postponed features:
  • newreleases apply does not just add resources, but can also update diverged resources
  • integrate configuration with package configuration
  • webhook configuration (might make sense to remove completely)