lib/ur: Implement crash (panic) reporting (fixes #959) (#5702)

* lib/ur: Implement crash (panic) reporting (fixes #959)

This implements a simple crash reporting method. It piggybacks on the
panic log files created by the monitor process, picking these up and
uploading them from the usage reporting routine.

A new config value points to the crash receiver base URL, which defaults
to "https://crash.syncthing.net/newcrash" (following the pattern of
"https://data.syncthing.net/newdata" for usage reports, but allowing us
to separate the service as required).

lib/config/migrations.go

@@ -25,6 +25,7 @@ import (
 // update the config version. The order of migrations doesn't matter here,
 // put the newest on top for readability.
 var migrations = migrationSet{
+	{29, migrateToConfigV29},
 	{28, migrateToConfigV28},
 	{27, migrateToConfigV27},
 	{26, nil}, // triggers database update
@@ -83,6 +84,19 @@ func (m migration) apply(cfg *Configuration) {
 	cfg.Version = m.targetVersion
 }
 
+func migrateToConfigV29(cfg *Configuration) {
+	// The new crash reporting option should follow the state of global
+	// discovery / usage reporting, and we should display an appropriate
+	// notification.
+	if cfg.Options.GlobalAnnEnabled || cfg.Options.URAccepted > 0 {
+		cfg.Options.CREnabled = true
+		cfg.Options.UnackedNotificationIDs = append(cfg.Options.UnackedNotificationIDs, "crAutoEnabled")
+	} else {
+		cfg.Options.CREnabled = false
+		cfg.Options.UnackedNotificationIDs = append(cfg.Options.UnackedNotificationIDs, "crAutoDisabled")
+	}
+}
+
 func migrateToConfigV28(cfg *Configuration) {
 	// Show a notification about enabling filesystem watching
 	cfg.Options.UnackedNotificationIDs = append(cfg.Options.UnackedNotificationIDs, "fsWatcherNotification")