allow setting max# or autoexpire pending administration

[Kenyee]

Some mailing lists gets spammed a lot, so the "tend to
pending administration work" gets huge after a few
weeks, especially if you forget about it for a while :-)

I'd suggest one of two things:
1) set a max# of pending admin items and autodelete the
ones that are oldest
2) set an automatic purge of items older than N days

I have a list I'm on where I can't even load the "tend
to pending admin work" page any more because it's so
huge. The browser just sits there and spins while the
server tries to send everything down. Of course, I've
neglected deleting spam (it's a list where only list
members can post) for a few months.... ;-)

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