Several improvements in the CERNSearch plugin

[pferreir]

We should be a bit smarter when notifying CERN Search about new Indico updates.

Some examples of new notifications:

• Attachment changes - do they need to re-download files?
• Specify whether a piece of metadata refers to a change or a new event/category/...;
• ACL changes - so that the serve updates it accordingly;

We should also include some extra information, like category names in addition to the ids.