Trim leading/trailing spaces in noticeable_nicks

This makes it possible to use a more humane comma-separation strategy, as for
example:

  "/set noticeable_nicks lamebot, boringbot, funnybot"

Also it's easy to inadvertently input spaces after a nickname:

  "/set noticeable_nicks da_bot "

as happens with tab-completion, and this change means it'll work just fine.