Epic: Improve diagnostics feature

[jandubois]

This epics builds on the work of #2319. It is explicitly not about adding additional diagnostics, which should be independent enhancement requests, but about improving the capabilities of the mechanism itself.

• Review diagnostics and develop guidelines for creating new ones #3064
• Design the "Fix action" UI
• Allow display of passing diagnostics
• Applying preference changes does not re-run diagnostics #3000
• Should network offline diagnostic be refreshed automatically #3005