Add util extension for calculating hot rank of comments

lib/util/hot_rank.dart

@@ -0,0 +1,15 @@
+// import 'dart:core' show ab;
+import 'dart:math' show log, max, pow;
+
+import 'package:lemmy_api_client/lemmy_api_client.dart';
+
+double _calculateHotRank(int score, DateTime time) {
+  final elapsed = (time.difference(DateTime.now()).inMilliseconds).abs() / 36e5;
+
+  // log instead of log10 in web version. does log == log10?
+  return (10000 * log(max(1, 3 + score))) / pow(elapsed + 2, 1.8);
+}
+
+extension CommentHotRank on CommentView {
+  double get computedHotRank => _calculateHotRank(score, published);
+}