Add Option to force players to either Plunder or Invade a Town

[ouckah]

Description:

This pull request aims to add an option to force players to either plunder or invade a nation. This is due to a possible exploit to plunder a nation and then invade them and tax them tons.

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New Nodes/Commands/ConfigOptions:

New Config Option: war.siege.switches.only_one_action_enabled
  - Default: false
  - If true, players can only either invade or plunder a town, not both.
  - If false, players can perform both actions.

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Relevant Issue ticket:

Closes #922

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

By making this pull request, I represent that I have the right to waive copyright and related rights to my contribution, and agree that all copyright and related rights in my contributions are waived, and I acknowledge that the TownyAdvanced organization has the copyright to use and modify my contribution under the SiegeWar License for perpetuity.

[LlmDl]

This PR appears to be missing the lang strings:

msg_err_town_already_plundered_only_one_action
msg_err_town_already_invaded_only_one_action

[ouckah]

@LlmDl added, are there any other config files that have to be updated?

[LlmDl]

No other changes are needed thank you.

[ouckah]

Tested and it's good to go! Thank you @LlmDl!