client: Explain Local Highlights vs. Remote

client: Explain Local Highlights vs. Remote

Add footer to "Local Highlights" page to note that local highlights
apply to the current device only, including a "Details..." button.

Add a "Details..." dialog with a brief explanation and points out
where to configure highlights for all devices.

For Monolithic builds, indicate that "Remote Highlights" replaces
"Local Highlights" to help avoid confusion.  There's no remote core,
so local highlights have no reason to exist other than to provide a
path for upgrades.