Favouriting Sessions should store in user meta

[dd32]

Now that users need to be logged into WordCamp.org for tickets, we should save user schedule favourites to their user meta in addition to using local storage.

That will allow for users favourites to be transferred to other devices by just signing in, rather than having to share the link from device 1 to device 2.

That will also introduce the ability for WordCamp organisers to know how many users have favourited a session.
As I'm not sure how many people actually use this functionality, it's unknown whether that would actually be useful to organisers.

One thing I'm not sure on, is the sharing favourites option:

1. Does it share the current data, as it currently does with a GET parameter?
2. Does it share the users favourited data, so you can see their favourites as they change?
3. Does it overwrite the opening users data? Merge?

My thinking currently is: It works as it currently does, GET param of session Ids, point-in-time data.
The Overwrite/merge is already a problem and "solved". You can't view saved sessions without overwriting your own:

You already have some sessions saved. Would you like to overwrite those with the shared sessions that you are viewing? [Cancel] [OK]