feat: top users per topic on topic detail

[stephancill]

The topic detail page should show some of the top users in the channel based on the amount of engagement that they get on their casts in this channel

This can be implemented in the form of a ?full=true query parameter on the /topic API endpoint and the introduction of a new TopicFull type which returns some extra details about a topic, only to be loaded on the topic's details screen.

The extra details in this case would be an array of User objects, topUsers, which is the top 5 users in the channel.

It can be displayed in the header of the topic detail in a way similar to the 'followers you know' component on the user profile detail/tooltip with the overlapping user profile images