Only publish data relevant to user
For performance and security reasons, restrict the publications to only publish documents for markets, contractsets, contrats and users that share tags with the logged in user.
Also the users do not need the entire activity log. Just the trades (for markets the user is tagged for) is sufficient.
We probably want some helper functions that return a set of tags for the logged in user, which can then be used with the $in directive in the published mongo queries.