Added a fix to prevent outpost exploit. Added a new flag war setting to only conquer outposts. Fixed negative upkeep

Current towny behaviour allows users to claim an outpost, and then claim townblocks adjacent to it as non-outpost townblocks. This makes them exploit the outpost limit, because they can simply disband the outpost, claim it as standard town block, and do it again and again with as many areas as they want. I simply changed the isEdgeBlock method to not count adjacent outpost townblocks as town borders.

The second change adds a new mechanic I enabled on my server, and I thought others could enjoy it: that's a simple config option to only enable players to place flags on outpost blocks.

The third fix is about negative nation upkeeps: the economy handler tried to withdraw a negative amount (not supported by vault - not tested on other economy providers) instead of depositing a positive one, and I fixed it