Unify cluster settings with general settings

[joaquimrocha]

Right now, as #1927 indicates, accessing settings is very confusing in Headlamp. To fix that, we want to have just one settings button, which will give access to the settings. The problem with this is that we have generic settings, cluster settings, and plugin settings.

So a quick fix for this for now is to add the cluster settings as part of the settings area. This means the sidebar would become like:

Settings
  General
  Plugins
  Cluster

This means that when a user clicks the Settings button at the top, it will do one of two things:

1. If we are in a cluster context, then we're taken to the Settings > Cluster.
2. If we are in a non-cluster context, then the user is taken to the Settings > General.

Now the problem is that if the user navigates into the Cluster settings from a non-cluster context, we need to know which cluster to display there, and that's why this Cluster settings view will have a cluster selector there instead of a cluster name. See the following mockup:

Note: If we only have one cluster set up AND we are in non-desktop mode, then we should show the cluster name as a title, not a selector (as it's unlikely to ever get more than one cluster).