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).

gui/default/syncthing/core/syncthingController.js

@@ -2426,4 +2426,9 @@ angular.module('syncthing.core')
             var err = status.error.replace(/.+: /, '');
             return err + " (" + time + ")";
         }
+
+        $scope.setCrashReportingEnabled = function (enabled) {
+            $scope.config.options.crashReportingEnabled = enabled;
+            $scope.saveConfig();
+        };
     });