Reduced the technical debt around our JS flash function by making it a
module that is imported rather than relying on the global function.
The global function still exists mainly for technical debt with how
some requests are being completed, but new JS should import the module
directly.

Also reduces some tech debt in the file by removing the need for jQuery.
Instead Flash is now 100% vanilla JS.

This reverts commit 68e40bd4, reversing
changes made to 2d1f823b.

This reverts commit 5e966688, reversing
changes made to b35378a9.

Previously it was listening for focusout, it now listens for blur &
paste events

Closes #28436

expand remaining non-explicit eslint-disable blocks and factor out globals when no-undef encountered