Web console: make retention dialog clearer

[vogievetsky]

Small UX tweaks to the retention dialog around how the cluster default rules are presented

Form mode:

JSON mode:

1. Render the default rules in the same way as the datasource rules just disabled
2. when switching the the JSON view the defaults turn to read only JSON also to show, intuitively, that they are the "same type of thing"
3. Added some explanatory text after the "Cluster defaults" label.

[suneet-s]

+1 after CI. Thanks @vogievetsky !