Stop people creating nether portals in claimed land

[Elioby]

Implemented as mentioned in #1160, stopping any player created portals from being built inside claims where they do not have the build permission.

Also added config option PreventNonPlayerCreatedPortals, which is disabled by default, which will stop any portals being created if the player who created it cannot be confirmed.

Closes #1160

[Jikoo]

LGTM 👍

Would probably be a lot faster to use logic similar to piston handling and create a bounding box from the portal blocks, but because I didn't extract the intersection logic like a big jerk it's not super easy without a larger diff.

[RoboMWM]

Interesting, wonder when this was added to that event.

Would you mind adding a section to the documentation describing this feature? https://github.com/TechFortress/GriefPrevention-Docs/blob/master/configuration.md

[Elioby]

Added documentation in this PR: GriefPrevention/GriefPrevention-Docs#3

[JHarris12345]

So is this being fixed in the spigot versions?