Fix DownloadManager, improve some network code

[mick-lue]

Problem

Issues with downloads getting stuck, not starting, or not correctly updating the download list.

Changes explained

downloadmanager

• remove duplicate includes
• Avoid dangling DownloadInfo in memory in createFromMeta
• explicitely set the redirect policy
• Do not create a meta file on download start, only on download end
• remove calling aboutToUpdate() when not using update(-1)
• Always connect the finished() signal from the NetworkReply so we don't miss it on very fast downloads (also check for fast finished downloads afterwards)
• the if statement in addNXMDownload was ignoring the first statement because of the comma
• add a simple fallback to getFileNameFromNetworkReply that uses the filename from the URL if it contains at least one char for the name and one char for the file extension
• generate random Download IDs instead of using the number of active downloads
  • QRandomGenerator is seeded the first time it gets used, so we don't need to do that ourselves
• rewrite downloadFinished to make it more consistent
• rewrite writeData so it does preliminary checks itself and the caller does not need to handle it
  • also removed m_hasData as it is unneeded in this setup

nexusinterface

• improved redirect handling
• add debug message for redirects

nxmaccessmanager

• solve deprecation of QWebSocket::error (https://doc.qt.io/qt-6/qwebsocket-obsolete.html)

organizer_en.ts

• As I added and edited translatable messages, I figured I'd include it.

My testing experience so far

I haven't had stuck downloads for the last 500 or so files. Before that, most files would instantly get stuck as my download folder was already quite full.

[mick-lue]

This is the last commit to this PR for now. I forgot to bring 2 changes over.