Improve usernames

[martinratinaud]

As the request for the user table can be very long on some searches let's

• display only the 1000 first
• rewrite bot repartition so that its calculation is now separated
• rewrite nb tweets repartition so that its calculation is now separated

A new major improvment could be to actually store directly in the users collection, the searches associated. I will try to do this in a near future