Add ability to watch/subscribe to a given talk

[terceiro]

Add a way for users to convey that they are interested in a talk. Actually, this could probably apply to anything in the schedule (and in that case we probably want #621 implemented first so that each item in the schedule has a fixed schedule item associated with it). This allows for at least 2 use cases:

• having a "my schedule" page where you get a list of what you want to attend
• being able to notify attendees when an activity is (re)scheduled