Only truncate a string if it is greater than the max length.

This fixes a bug where a string like "exact length string truncated"
(length of 29) is truncated to
```php
    truncate_string("exact length string truncated", 29, "...")
        = "exact length string truncated..."
```
a string that is both longer than 29 characters and rather strange
looking.
Make sure that the length of a truncated string does not exceed $max_len.

A special case for '&hellip;' is used, as it is only one character wide
when display.
Try to use larger GoComics image
Merge branch 'gocomics_large'
Merge branch 'truncate_string'