rename 'report_persistence' capability to 'historic_reports'

since that better captures the intent - the UI doesn't care about
reports get stored, but what reports it can retrieve.

common/xfer/constants.go

@@ -14,9 +14,9 @@ const (
 	ScopeProbeVersionHeader = "X-Scope-Probe-Version"
 )
 
-// ReportPersistenceCapability indicates whether probe reports end up in a
-// long-term storage and can be retrieved.
-const ReportPersistenceCapability = "report_persistence"
+// HistoricReportsCapability indicates whether reports older than the
+// current time (-app.window) can be retrieved.
+const HistoricReportsCapability = "historic_reports"
 
 // Details are some generic details that can be fetched from /api
 type Details struct {

prog/app.go

@@ -295,7 +295,7 @@ func appMain(flags appFlags) {
 	}
 
 	capabilities := map[string]bool{
-		xfer.ReportPersistenceCapability: flags.s3URL != "local",
+		xfer.HistoricReportsCapability: flags.s3URL != "local",
 	}
 	handler := router(collector, controlRouter, pipeRouter, flags.externalUI, capabilities)
 	if flags.logHTTP {