This mistake seems to have always been there, but it only resulted in
errors on the `/explore*.json` since they were the one that _actually_
relied on the local variables.

* 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

As discussed in https://github.com/mbleigh/acts-as-taggable-on/issues/91,
we can avoid N+1 queries if we use `tags` instead of `tag_list`.

Seen while reviewing
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19740.

Make Admin::UsersController work with Ruby 2.6

See merge request gitlab-org/gitlab-ce!25316

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

Ruby 2.6 introduced `Enumerable#filter`, which takes no arguments.
Attempting to call `filter` on an `ActiveRecord::Relation` with a scope
will fail with a `wrong number of arguments (given 1, expected 0)`
message because the `Enumerable#filter` implementation overrides the
delegated `ActiveRecord::Relation#filter` method.

To make Admin::UsersController compatible with Ruby 2.6, rename
`User.filter` to `User.filter_items`.

Previously, if you entered a username in the URL manually to view a
specific user, the lookup was done case-sensitively, despite usernames
being case-insensitive, often resulting in a 404. We now use the same
`find_routable!` logic as the non-admin Users controller.

By default in Rails 5, content passed to `render` will be
escaped. This doesn't work for the HTML profile output, which
should be considered safe HTML already.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56152

Updates specs to use new rails5 format.

The old format:
`get :show, { some: params }, { some: headers }`

The new format:
`get :show, params: { some: params }, headers: { some: headers }`

Adds gitlab.impersonation_enabled config option defaulting to true to
keep the current default behaviour.

Only the act of impersonation is modified, impersonation token
management is not affected.