- Update PO file

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.

Show header and footer system messages in email

Closes #56863

See merge request gitlab-org/gitlab-ce!25474

* 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

Due to performance reasons we cannot use the type filter on the tags.
The table for ActsAsTaggableOn is too big and too unoptimized, such that
the queries time out on production.

See the discussion
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19740#note_120087938
for more info.

the potential number of available runner tags is too large to load it
statically to a dropdown. we use the same lazy loaded dropdown as is
used for the users dropdown already.

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`.