don't overshorten slugs (bug 971323)

This fixes [bug 971323](https://bugzilla.mozilla.org/show_bug.cgi?id=971323)

The key is to not "over" shorten the slug if there's no clash, and leave it 30
chars long.

Only if a clash is detected, we shorten it to 27 chars (instead of 30) to leave
space for a dash and a two digits long number, eg ```-13```.