[FIX] website_forum: no limit on user post list for moderators

The limit on the list of answers and questions posted by
a given forum user is purposely limited to reduce the
performance penalty for displaying them all.
(see 78fa8619)

However seeing the full list is useful for forum moderators
(e.g. when tracking down abuse), and there are only a few
such users with high karma, so enabling it for them is
negligible performance-wise.

Fixes #3955