Delay langpack installation to end of GIS flow

Lang pack installation was being attempted before the wife page was shown. This changes removes attempting the download from the langauge chooser.

Instead, a spinner and prompt is shown on the summary page, where the download is now being done. Once complete, the user can start using Ubuntu.

They can optionally cancel the download and proceed to Ubuntu. No download is attempted if there is no active network connection.

Additional fixes an issue we were encountering with pkexec:

As of 24.10, systemd is no longer compile for Ubuntu using the -Dbump-proc-sys-fs-nr-open=false flag. The absence of this flag causes systemd to set file descriptor limits to their maximum.

24.04 LTS: https://git.launchpad.net/ubuntu/+source/systemd/tree/debian/rules?h=ubuntu/noble#n82

24.10: https://git.launchpad.net/ubuntu/+source/systemd/tree/debian/rules?h=ubuntu/oracular

This was causing pkexec to sit attempting to close 1073741816 file descriptors which took several minutes everytime it was called to install a lang pack.

This change uses ulimit to set the max file descriptors to a reasonable max to avoid this behaviour, making use of the existing wrapper script we already have patched in.