Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38871

changed how the container class is added onto the text element
more specs

added back in documentation comment to flash file

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.