This reverts merge request !26430

There is nothing more to say but

"Commits that change 30 or more lines across at least three files
must describe these changes in the commit body"

This removes extraneous newlines when copying comments
with an ordered list.

The spec needed to be updated because in some cases the resulting
Markdown is slightly different, though equally valid.

The schema is built on top of the default schema and Node and Mark
classes provided by tiptap-extensions. prosemirror-model is used to
parse HTML/DOM into a prosemirror document, and prosemirror-markdown is
used to serialize this document to (GitLab Flavored) Markdown.

Used in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110
which introduces a dynamic variable list and needs secrets.

We use the valueSelector/placeholderSelector options to avoid selecting
the last empty row.

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/38019

Tested on the following platforms that split the emoji into separate
characters currently.
 - Windows 10 FCU
    - Chrome 62.0.3202.89
    - Firefox 57.0b14
    - MS Edge 41.16299.15.0
 - macOS 10.12.6
    - Chrome 63.0.3239.40
    - Safari 11.0.1 (12604.3.5.1.1)
    - Firefox 57.0
 - Ubuntu 16.04
    - Chromium 62.0.3202.75
    - Firefox 56.0

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

Resolve "Submitting reply to existing diff discussion using Cmd/Ctrl+Enter submits twice and refreshes page"