Improved Banner Thousands Separator Logic

Use `number_format` to generalize inserting the thousands separators.
This fixes formatting for distances over 1 million.

Tested numbers up to 1 billion.

| Old format   | New format     |
|--------------|----------------|
|            1 |              1 |
|        1,000 |          1,000 |
|     1000,000 |      1,000,000 |
|  1000000,000 |  1,000,000,000 |

Fixes #153.

Side notes:

1. We specify "," because the banner is in English. We could load the
   user's locale and use that but that would only make sense if we
   internationalized the whole banner.
2. The web UI doesn't have any thousands separator at all. Easy enough
   to add but ought to be done in a separate PR.

badge/banner.php

@@ -79,7 +79,7 @@ function renderImage($textLines, $user = false) {
         $units = "miles";
     }
     $flights = sprintf("%s flights", $row["count"]);
-    $miles = sprintf("%d,%03d %s", $distance / 1000, $distance % 1000, $units);
+    $miles = sprintf("%s %s", number_format($distance, 0, ".", ","), $units);
     $duration = sprintf(
         "%d days, %d:%02d hours",
         $row["duration"] / 1440,