Add Global plugin - Githubissues

[x] Redirecting to new events
[x] Read-only mode + message for Indico Global category
[ ] CLI for various notifications
[ ] CLI to mark exported categories/events as deleted?
[ ] Rename plugin (to what?) to make it clearer it runs on the main Indico instance and not on Indico Global, especially in case we may need a custom plugin on Indico Global at some point.

indico / indico-plugins-cern