It's very easy to forget to add `require 'rails_helper'` at the top of every core/plugin spec file, and omissions can cause some very confusing/sporadic errors.
By setting this flag in `.rspec`, we can remove the need for `require 'rails_helper'` entirely.
Previously we were publishing one messagebus message per user which was 'tracking' a topic. On large sites, this can easily be 1000+ messages. The important information in the message is common between all users, so we can manage with a single message on a shared channel, which will be much more efficient.
For user-specific values (notification_level and last_read_post_number), the JS app can infer values which are 'good enough'. Correct values will be loaded as soon as a topic-list containing the topic is visited.
