The lxml.etree.fromstring() function use the default XML parser, which resolves
external entities because XML handling defaults in Python sucks. There is no
need for such dangerous misfeatures in calibre-web, so let's disable it.
A user able to upload epub/fb2 could add something like this to the file:
```xml
<?xml version="1.0"?>
<!DOCTYPE foo [<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
<container><rootfiles><rootfile full-path="&xxe;"/></rootfiles></container>
```
and obtain the content of the `/etc/passwd` file, which is bad™.
