At the time of writing, this is how the `TopicPosterSerializer` looks
like:
```
class TopicPosterSerializer < ApplicationSerializer
attributes :extras, :description
has_one :user, serializer: PosterSerializer
has_one :primary_group, serializer: PrimaryGroupSerializer
has_one :flair_group, serializer: FlairGroupSerializer
end
```
Within `PosterSerializer`, the `primary_group` and `flair_group`
association is requested on the `user` object. However, the associations
have not been loaded on the `user` object at this point leading to the
N+1 queries problem. One may wonder
why the associations have not been loaded when the `TopicPosterSerializer`
has `has_one :primary_group` and `has_one :flair_group`. It turns out that `TopicPoster`
is just a struct containing the `user`, `primary_group` and
`flair_group` objects. The `primary_group` and `flair_group`
ActiveRecord objects are loaded seperately in `UserLookup` and not preloaded when querying for
the users. This is done for performance reason so that we are able to
load the `primary_group` and `flair_group` records in a single query
without duplication.
User flair was given by user's primary group. This PR separates the
two, adds a new field to the user model for flair group ID and users
can select their flair from user preferences now.
