The $sidebar_refresh_time option was removed with the changeset
1f840760e6e0. Remove it from the sample sidebar configuration as well,
so that anyone using that file in its entirety won't have to deal with "unknown
variable" errors.
# Display the Sidebar mailboxes using this format string.
set sidebar_format = '%B%?F? [%F]?%* %?N?%N/?%S'
-# Sidebar will not refresh its list of mailboxes any more frequently than
-# this number of seconds. This will help reduce disk/network traffic.
-set sidebar_refresh_time = 60
-
# Sort the mailboxes in the Sidebar using this method:
# count - total number of messages
# flagged - number of flagged messages