WIP: Aggregated view

[joaquimrocha]

This is a preview of the aggregated views I have been working on.

We need to test this thoroughly to make sure no regressions or deep problems are added. I'd like everyone testing to pay special attention to spotting any performance issues, since a lot more requests are now happening.

How to test:

1. Go into a cluster view and edit the URL to be the name of multiple clusters separated by +, example minikube+mycluster1+mycluster2
2. Notice that the version number (bottom left) now displays either 1 version (if all clusters have the same version), 2 versions (if only 2 different versions exist among the clusters), or version+... if more than 2 different versions exist among the clusters.
3. When in a multi-cluster context, verify that list views have a cluster column
4. When not in a multi-cluster context, verify that list views don't have a cluster column
5. Home (/) should a list of all the clusters, whether they can be accessed or not
6. Cluster groups can be created from the home view
7. MISSING: Cluster groups are shown in the list of clusters
8. Cluster group settings can be accessed from the same user menu as when in a regular cluster
9. MISSING: The cluster group settings link in the user menu has the label: Group Settings, instead of Cluster Settings.
10. The cluster settings link in the user menu has the label Cluster Settings.