Signed-off-by: Rémy Coutable <remy@rymai.me>

fixed scss-lint

Closes #40041

spec fixes

clears all tmp files after cancelling edit mode

Closes #38614

Adds a test for flattenedFiles
Changes the data method to not be an arrow method
Various other review fixes

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.