Squarify world borders

[MilesBHuff]

While the round world borders added in #179 look great on Dynmap, nearly perfectly following many terrain features; the sad reality is:

• We are dependent upon a very out-of-date plugin that could break at any moment. (https://github.com/PryPurity/WorldBorder)
• Should this plugin break, chunks outside that round border will become visitable, which will result in chaotic terrain after we update to 1.18 and higher.
• When a player reaches this plugin's world border, they are erratically jerked back to some square from before they crossed, which is unpleasant and confusing.
• Chunks beyond this plugin's world border are not unloaded in the way that they are past the vanilla world border.

Because of all these considerations, I think it is best that we switch to the vanilla world border, even though it is perfectly square and does not align nearly as well with our world.

[MilesBHuff]

Depending on how this is done, it may be possible to mask #196.

[MilesBHuff]

This has been done.