Server script to download the latest installer

As promised, I talked with enumag about the script and he sent it to me earlier today.

You can access it at http://07th-mod.com/installer.php

By default, it will guess the user platform and get the latest stable release. You can force it to go through manually using: ``http://07th-mod.com/installer.php?system=win|mac|linux&branch=pre|stable``

Example: http://07th-mod.com/installer.php?system=linux&branch=stable

Beware that using ``branch=pre`` will only work if the most recent release is marked as a pre-release. If the most recent is a stable release, it will get the stable instead of the pre-release.

Right now the guessing part doesn't work, but I believe it is because of the current changes ongoing at the releases page. Earlier today it was working fine, so it might fix itself soon enough.

该脚本托管在 http://07th-mod.com/installer.php 并与 GitHub 发布版本交互。首先检查现有的服务器端逻辑，用于平台检测和发布版本获取。了解它如何确定 'stable' 与 'pre' 分支，并与项目的发布页面集成。使用不同的查询参数测试端点，以验证当前行为并识别猜测问题。

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