Feature: watch later

[ilya-g]

KotlinConf gives a plenty of interesting talks, so that it's impossible to watch them all being at the place.

It would be nice to mark the talks that I want to watch after the conference when their videos become available and to get a (single) notification about that fact.

Currently we have a flag that represents a bookmarked talk, but it's impossible to distinguish the talks I already watched and those that I still want to watch.