Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64454

https://gitlab.com/gitlab-org/gitlab-ce/issues/58583

This is one of many MRs opened in order to improve the overall
internationalisation of the GitLab codebase.

This commit only targets Vanilla JS files.

i18n documentation
https://docs.gitlab.com/ee/development/i18n/externalization.html

Partially addresses #47006.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/41563

`event.srcElement` is non-standard,
https://developer.mozilla.org/en-US/docs/Web/API/Event/srcElement

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.