升级系统到最新的 MovableType R5001

此次更新算是大版本的更新，R5001，（7.8.0）。

这些版本包括一些变化、错误修复和安全修复。

在更新时，强烈建议你在更新前对数据库进行快照，然后在不同的目录下安装Movable Type，而不是覆盖你当前的安装。

不管怎样，我都是直接覆盖安装的。当然有做备份，这还是必须的。

新的和改进的功能

[MTC-27870] Improved the performance of rebuilding MTEntries tag with category, tag and field modifiers [MTC-27869] Improved the performance of rebuilding MTAssets tag with tag modifier [MTC-27839] Added a tool to remove unnecessary data from mt_plugindata: tools/remove-broken-plugindata [MTC-27823] Added a tool to remove sites without parent_id: tools/remove-parent-missing-sites [MTC-27814] Added a tool to remove unnecessary revisions: tools/reduce-revisions [MTC-27810] Removed broken data automatically in the list views of revision histories of Entry, ContentData and Template if exist (MT7) [MTC-27004] Added to log the activities of updating Movable Type itself [MTC-27930] New configuration directive: ReferrerPolicy to control the value of Referrer-Policy header [MTC-27926] Fixed to enable to reload Captcha images [MTC-27897] Fixed to work tools/utf8mb4_converter with utf8mb3 encoding [MTC-27896, 27895] Improved quality and size of Captcha images [MTC-27894] Improved a message about Google Analytics setting.