Common README files for IDP users

**is your feature request related to a problem? Please describe.**
I'd like to add a read-only README.md to the private upload folders of IDP accounts. this config snippet:
```
[/users/${u}/private/README.md]
/srv/files/README.md
accs:
r: *
```
produces this error:
```
ERROR: if ${u} is mentioned in a volume definition, it must be included in both the filesystem-path [/srv/files/README.md] and the volume-url [/users/${u}/private/README.md]
```
which is understandable but unfortunate.

**Describe the idea / solution you'd like**
it makes sense that you can't normally have Quantum Magic Volumes like this, but ideally copyparty should be smart enough to suppress the warning if the volume is read-only and therefore cannot fall victim to concurrent modification shenanigans

**Describe any alternatives you've considered**
I could probably modify my instance's theme, but that sounds like way more work than opening a github issue :3

检查卷配置的解析和验证逻辑，这可能位于处理 Quantum Magic Volumes 的模块中。错误消息表明需要检查变量 ${u} 的使用在哪里得到验证。首先在代码库中搜索 'ERROR: if ${u} is mentioned'，以定位相关的验证函数。然后检查只读标志的处理方式，以及它们是否可以抑制警告。测试包括使用所述配置创建一个卷，并验证在只读时错误会被抑制。

由索引模型根据 Issue 内容生成。