Use `ignore_user_abort()` to avoid some update failures

[dd32]

As mentioned in 5:ticket:16066 and 5:ticket:29679 we should consider using ignore_user_abort() to prevent the potential issue of an upgrade process being aborted mid-way due to a browser cancellation.

This could be called before any update activity starts, or perhaps only after actual changes start happening (ie. after the zipfile download, before the file alterations).

[afragen]

I will attempt to reproduce this issue.