Add Webm medium quality + fix MP4 high quality presets

Currently we have access to a low quality option, and the default of high quality, but something in the middle feels like it'd be better for backup uses, like the same options as high quality, but limited to 720/1080p instead of the highest quality available

Even just an option to set the max download resolution could work to resolve this

该 issue 未指出任何文件或测试。首先定位现有的 WebM/MP4 低质量和高质量预设定义及其测试覆盖情况，然后明确中等质量应当是一个预设还是最大分辨率选项。完成的标准是：为 WebM 和 MP4 定义所选质量行为，并由测试覆盖。