Autocontinue constructing farms when the first queued for construction is full.

Seen at around minute 20 at https://wildfiregames.com/forum/topic/15271-0-ad-on-youtube/page/114/#comment-544694.
If not tasked by the user, entities will not autogather. But now they at least construct the farms.

Differential revision: https://code.wildfiregames.com/D4945
Comments by: @phosit, @Stan