Revamp the announcements system.

This commit transforms the announcements system in the following way:
- announcements.yml makes it easier to add missing nodes and remove
  obsolete ones. This way, users don't have to delete their previous
  announcements-file to get the new keys.
- Color codes are supported in the fact that when an announcement
  message is "set()", the string value has all of its &-symbols
  translated.
- No need for a defaults-file, because the enum is very easy to
  compile into a YAML file.
Fix typo in error message.