Publicize personal calendars on talk knowl

[edgarcosta]

At the moment in the talk knowl we have 3 links

 Export talk to    Google Calendar    iCal/Outlook    ICS file

but we don't give any indication to the non-logged in user that the simplest to way to add an event to his personal calendar is by registering, and save the talk to its favorites.

we should try to make this more clear.

ps: to be honest, I don't see the point of having a calendar for a single talk, but if we remove those links people will again ask for them

[poonen]

Actually, I wrote a fix for this already in #578, and now I've made the knowl text recommend registering even more strongly. The knowl text is already in knowls.yaml; it's just waiting for someone to implement it.