Allow logged-in users to like sessions in the listing

[tylerdave]

Once we've published the list of talks I'd like to give users the ability to "like" or "favorite" talks in the list so we can gauge relative interest and use this information when building the schedule (especially for room size selection.)

[tylerdave]

If this doesn't make sense to integrate in symposion for this year, we could simply use an online survey form once the talks list is pubilshed.