* Add email_header_and_footer_enabled flag to appearances table
* Set email_header_and_footer_enabled default value to false
* Add checkbox to appearance to toggle show header and footer in emails
* Add email_header_and_footer_enabled to allowed params in controller
* Add header and footer messages to the html and text email layouts
* Remove the color styling for emails header and footer
* Add empty_mailer layout for emails without layout,
to have the header and footer applied

remove EE specific code
remove EE licence checks
move migration from EE to CE folder structure
move specs from EE to CE folder structure
remove EE specific flag specs

Same as `branded_login_page.md`

For the sake of consistency, removes any extraneous
consecutive blank lines from the doc suite.

The pngquant tool was used https://pngquant.org, and particularly, the
following command:

        /usr/bin/pngquant -f --skip-if-larger --ext .png --speed 1 image.png

Before: 47584K
After : 34924K

Took it from https://gitlab.com/gitlab-com/www-gitlab-com/merge_requests/3232

[ci skip]

Split into user and administrator docs:

- administration/issue_closing_pattern.md
- user/project/issues/automatic_issue_closing.md

[ci skip]

[ci skip]

The only major difference with the EE version is the change from a light and dark logo to only a header logo
The dark logo wasn't used anyway, so it seemed to make sense to me to rename the field to the actual function of it

Added phrase "Here's how to close multiple issues in one commit message" so that doc is easier to find by users

capitalized.

Make the plural version consistently "URLs".

Fix an instance where the article "the" before URL was missing.