Add the ability for nations to sanction towns, preventing them from joining the nation via invite or join.

[LlmDl]

Description:

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New Nodes/Commands/ConfigOptions:

  - New Command: /nation sanctiontown
    - add [Townname] - Adds a town to the sanctioned town list.
    - remove [Townname] - Removes a town from the sanctioned town list.
    - list - Lists your nation's sanctioned towns.
  - New Command: /ta nation [nationname] sanctiontown
    - add [Townname] - Adds a town to the sanctioned town list.
    - remove [Townname] - Removes a town from the sanctioned town list.
    - list - Lists the nation's sanctioned towns.
  - New Permission Node: towny.command.nation.sanctiontown
    - Allows adding/removing sanctioned towns, a child node of towny.command.nation.*.
    - No townyperms.yml change is required.
  - New Permission Node: towny.command.townyadmin.nation.sanctiontown
    - Allows admins to add/remove sanctioned towns from a given nation, a child node of towny.command.townyadmin.nation.*.
    - No townyperms.yml change is required.
  - API: Added NationSanctionTownAddEvent
    - A cancellable event which is fired before a town becomes sanctioned.
  - API: Added NationSanctionTownRemoveEvent
    - A cancellable event which is fired before a town becomes unsanctioned.

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Relevant Towny Issue ticket:

Closes #6544.

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• I have tested this pull request for defects on a server.

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