This reverts merge request !14768

We were looking these up on each request to an issue page, because the form is
pre-filled, as is the template dropdown. That was unnecessary: we could just
treat these as 'special' repository files (like the rendered README) and cache
them in Redis until they change on a push.

The basename appears to have been a holdover from the past - it doesn't look
necessary now. Some of the regexes were unanchored on one side, so explicitly
ensure that they only match the root.

Apart from that, this means that pushing to foo/README.md will no longer
invalidate the main README cache for a project.

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

This also refactor the email_helper support spec to watch for multiple
emails being sent.

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.