Content Editor is stripping new lines between lists and tables

Summary

As reported in #332629 (comment 863199937), the Content Editor is stripping blank lines between unordered lists and tables. This results in improperly formatted Markdown and the table doesn't render.

Steps to reproduce

1. Create a wiki page with the following code in raw markdown:

   1. - 1
      - 2
      - 3
      
      | header | header | header |
      | ------ | ------ | ------ | 
      | cell | cell | cell |
      

2. Edit that page in the Content Editor by clicking the Edit rich text link above the editor
3. Save the page in the Content Editor
4. Observe the new formatting of the page is incorrect, with no blank line between the end of the list and the table.

Example Project

What is the current bug behavior?

The Content Editor strips out the newline

What is the expected correct behavior?

Tables should always be preceeded and followed by blank lines

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes