This commit moves the rendering of the button back to the server, and
shows/hides it using opacity rather than display. It also removes the
transform applied to the button on hover (scale). Previously, both of these
factors automatically triggered a reflow, which creates a performance
bottleneck on pages with larger DOM size.

MR: !12103

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33868

While in a MR "Changes" tab looking at some diff note discussions.
Clicking reply and pressing the up-arrow should edit your last comment in
that discussion.

I suspect this regressed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11173/diffs#f4bb984ec495c5cb58516785c62978f5209c39b4_179_178
where we changed the selector

Always use a explicit true/false value to set highlight because jQuery
assumes `null` means toggle from whatever state and doesn't force false.

Regressed in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12098

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/27070

Deprecate "chat commands" in favor of "slash commands"

We looked for things like:

 - `slash commmand`
 - `slash_command`
 - `slash-command`
 - `SlashCommand`

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33529

Bug introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11278

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/33483

Escapes html content before appending it to the DOM

See merge request !2107

Ran:
 - git format-patch v9.2.2..v9.2.5 --stdout > patchfile.patch
 - git checkout -b 9-2-5-security-patch origin/v9.2.2
 - git apply patchfile.patch
 - git commit
 - [Got the sha ref for the commit]
 - git checkout -b upstream-9-2-security master
 - git cherry-pick <SHA of the patchfile commit>
 - [Resolved conflicts]
 - git cherry-pick --continue

Ran:
 - git format-patch v9.2.2..v9.2.5 --stdout > patchfile.patch
 - git checkout -b 9-2-5-security-patch origin/v9.2.2
 - git apply patchfile.patch
 - git commit
 - [Got the sha ref for the commit]
 - git checkout -b upstream-9-2-security master
 - git cherry-pick <SHA of the patchfile commit>
 - [Resolved conflicts]
 - git cherry-pick --continue

Fix up tech debt comments from @jschatz1

 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10837#note_28757436
 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10837#note_28757905
 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10837#note_28757952
 - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10837#note_28758013

This reverts merge request !2105

Command | Result
$link.data('lineType') | ''
$link.data('line-type') | ''
$link.attr('data-line-type') | 'new'
$link[0].dataset.lineType | 'new'

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32551

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32449

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32424

Bug introduced in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/11327

Flash error messages disappear(`fadeOut`) automatically after any successful request

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32125

Closes #31348