Improve way of storing user lists

[loicknuchel]

Groups, events, talks and proposals have a list of users linked (orgas, talks or speakers). The current way of doing this is quite ugly: we concatenate all ids with a ',' and save them to a string column.

We should investigate ways of keeping the foreign key on them, maybe using the postgres array type, or maybe using a join table. But this probably require a lot of query rewrites :(