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

Closes #33896

Fix inspired while working on
https://gitlab.com/gitlab-org/gitlab-ce/issues/32801

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32801

change all .nav-links to use flex
place Resolve Discussions above tabs on small screens, and to the right on large

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/30810

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/30863

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/29414