Revert "Merge branch '44870-remove-extra-space-around-comment-form-on-merge-requests' into 'master'"

This reverts merge request !18120

Signed-off-by: Takuya Noguchi <takninnovationresearch@gmail.com>

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/41491

Closes #41561

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/39608

Use SVG icons to avoid having to position things
See https://gitlab.com/gitlab-org/gitlab-ce/issues/39608#note_50088917

Closes #36687

Closes #37589

Closes #36479

added specs to sticky util file
added `No files found.` text when results are empty

This makes navigating through diff files quickly. Currently we just
toggle a list, which could be pretty big. This moves it into a dropdown
to make it much easier.

Also includes a filter bar to quickly search for certain
files/extensions.

Closes #29778

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