Parse html with rust

This prevents us from having to use QTextDocument to parse untrusted html
emails. Resulting in crashes or worse security issues.

QTextDocument is still used in case corrosion is not found

Example of crash this prevents:

```
 #5  0x00007f0e8a3e23f4 in QPalette::QPalette(QPalette const&) () at /lib64/libQt5Gui.so.5
 #6  0x00007f0e8a4cb8ff in QTextHtmlParserNode::applyCssDeclarations(QVector<QCss::Declaration> const&, QTextDocument const*) () at /lib64/libQt5Gui.so.5
 #7  0x00007f0e8a4ce2d9 in QTextHtmlParser::parseTag() () at /lib64/libQt5Gui.so.5
 #8  0x00007f0e8a4ce958 in QTextHtmlParser::parse() () at /lib64/libQt5Gui.so.5
 #9  0x00007f0e8a4f39a2 in QTextHtmlImporter::QTextHtmlImporter(QTextDocument*, QString const&, QTextHtmlImporter::ImportMode, QTextDocument const*) () at /lib64/libQt5Gui.so.5
 #10 0x00007f0e8a4b1868 in QTextDocument::setHtml(QString const&) () at /lib64/libQt5Gui.so.5
 #11 0x000000000041ec22 in EmailIndexer::processPart(KMime::Content*, KMime::Content*) (this=...