Improve indication of API-exportable events

When I look at a (protected) category in Indico, there are (at least) three types of events, which are listed:
- events that I can see (i.e. inspect, open the event and look at more details)
- events that I cannot see
- because they are protected with an access key
- because they are protected with another ACL (which I am not part of)

When I use the API + signature link URL to export the iCal file for the category, then only the events I "can inspect" are exported, which are sometimes much less than those I see in the list of the category page.

Proposal: Indicate in the category display, if the events are (will be) visible for me, when I try to open them. Presently there is already the indication protected/unprotected, but that is not really enough (any more, due to new and better protection mechanisms).

Start by tracing the category display and the API signature-link iCal export described in the issue. Determine how protected, inaccessible events are currently distinguished from visible events, then verify that the category view indicates whether each event can be opened and that the result matches export behavior.

Written by the indexing model from the issue text.