Figuring out what unread topics a user has is a very expensive
operation over time.
Users can easily accumulate 10s of thousands of tracking state rows
(1 for every topic they ever visit)
When figuring out what a user has that is unread we need to join
the tracking state records to the topic table. This can very quickly
lead to cases where you need to scan through the entire topic table.
This commit optimises it so we always keep track of the "first" date
a user has unread topics. Then we can easily filter out all earlier
topics from the join.
We use pg functions, instead of nested queries here to assist the
planner.
* cut down on storage of the work Topic, 3 times per row (in 2 indexes)
* only store one view per user per topic
* only store one view per ip per topic
Introduced badge triggers, introduced concept of badge that happens due to a post but has the post hidden
Delta badge grant happens once a minute, backed by redis
This is information that is not usually needed when representing a user
and is in a separate table with a has one relationship to avoid querying
it all the time.
