This ports the changes for the CODEOWNERS feature to CE:
- It adds the CODEOWNERS file.
- It adds the mention of the `with-codeowners` branch in TestEnv

Signed-off-by: Rémy Coutable <remy@rymai.me>

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The issue template for bugs has a notice at the start that mentions to
search for duplicates before creating a new issue. Then the reporter
is asked to remove the notice once read, presumptively to avoid noise
in the actual issue.

Instead of that, we can make that part of the issue template an html
comment, so it won't be rendered in the final result. This avoids
the tedious task of removing that part of the notice.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43580

Signed-off-by: Rémy Coutable <remy@rymai.me>

This removes some somewhat redundant checkboxes from the template and
rephrases the introduction now that those settings are in GDK.

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Signed-off-by: Rémy Coutable <remy@rymai.me>

This template includes a rather large (but fairly easy to follow)
checklist. This checklist should hopefully take away quite a bit of work
from database specialists by requiring authors to provide all the
necessary info.

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* Bugs reported in the GitLab Issue trackers should be reproducible
* Asking reporters to reproduce on GitLab.com is reasonable
  * It lets us know whether the issue still exists in latest
  * It ensures that the steps to reproduce are solid
  * It gives Developers a live example to work from
  * Reporter can verify the fix in the example project once shipped

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Signed-off-by: Rémy Coutable <remy@rymai.me>

Triple backticks aren't allowed inside <details> elements, so add an explicit
<pre> instead.

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Signed-off-by: Rémy Coutable <remy@rymai.me>