Configurable Nether Portals

Adds 5 new gamerules:

Name	Default	Description
`portalsAllowCryingObsidian`	`true`	Whether portals can be created using crying obsidian.
`portalsAllowCustomShapes`	`true`	Whether portals can have non-rectangular shapes.
`portalsCustomSearchMaxDepth`	`512`	How many blocks will be searched for a valid portal, before giving up. Only used if `portalsAllowCustomShapes` is `true`.
`portalsCustomShapeMinBlocks`	`1`	The minimum number of portal blocks needed to be a valid portal. Only used if `portalsAllowCustomShapes` is `true`.
`portalsCustomShapeMaxBlocks`	`512`	The maximum number of portal blocks needed to be a valid portal. Only used if `portalsAllowCustomShapes` is `true`.

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