[APM] UI Indices configuration: Revisions to CCS design

[formgeist]

Summary

Based on some feedback in the implementation issue, we're putting another round of revisions to the design to accommodate the following improvements;

• only show "Add Cross Cluster Indices" link if at least one remote cluster has been added
• warn the user if they manually type :apm- without having added any remote clusters
• warn the user if they manually type mycluster1:apm-* if they only have a remote cluster named mycluster2.
• Similarly we could add a warning if no indices matches the specified pattern.

[formgeist]

@elastic/apm-ui Putting this in the backlog in design because I don't believe we're prioritizing doing any work on the CCS for the coming releases.