Rework brokers page

[weeco]

The brokers page currently does not provide a lot of additional value besides inspecting broker configs. This issue suggest to change a few things to rework the brokers page:

1. Each broker should get it's own details page (similar like topics), so that we can add more info per broker
2. Besides configuration we can show the hosted partitions / replicas and their in-sync status (including the ISR state of the replicas)
3. Show additional stats such as under replicated partitions for this broker, to identify issues quickly
4. Add a tab that allows to inspect the used log dir size on the broker for each topic & partition
5. Show "offline" brokers

[cpflaume]

This would be a useful feature, as it allows to get an overview about what is happening/running on a certain broker. Especially 2. would be nice to see the distribution of partitions to a broker.

[cruscio]

While this is already covered in the description, just want to voice interest (and reiterate without typo replciated , so it'll appear in search):

It would be great if this rework included under replicated partitions (e.g. kafka-topics --describe --under-replicated-partitions) so one could observe the status of recovery while rolling broker updates.