Adds noref/noopen to external links for Mailgun and Snowplow

What does this MR do and why?

Adds rel="noopener noreferrer" to external links for Mailgun and Snowplow in the admin/settings/general menu.

This is a best practice to avoid "Reverse Tabnapping".

Screenshots or screen recordings

This was surfaced by running an authenticated DAST active scan on a self-managed GitLab instance.

How to set up and validate locally

1. Navigate to admin/settings/general in GitLab instance

2. Inspect links for Mailgun and Snowplow, note that they have target="_blank" but do not have rel="noopener noreferrer".

3. Edit _mailgun.html.haml:11 and _snowplow.html.haml:10

   app/views/admin/application_settings/_mailgun.html.haml:11

   --      = _('Configure the %{link} integration.').html_safe % { link: link_to(_('Mailgun events'), 'https://documentation.mailgun.com/en/latest/user_manual.html#webhooks', target: '_blank') }
   ++      = _('Configure the %{link} integration.').html_safe % { link: link_to(_('Mailgun events'), 'https://documentation.mailgun.com/en/latest/user_manual.html#webhooks', target: '_blank', rel: 'noopener noreferrer') }

   app/views/admin/application_settings/_snowplow.html.haml:10

   --       = html_escape(_('Configure %{link} to track events. %{link_start}Learn more.%{link_end}')) % { link: link_to('Snowplow', 'https://snowplowanalytics.com/', target: '_blank').html_safe, link_start: link_start, link_end: '</a>'.html_safe }
   ++       = html_escape(_('Configure %{link} to track events. %{link_start}Learn more.%{link_end}')) % { link: link_to('Snowplow', 'https://snowplowanalytics.com/', target: '_blank', rel: 'noopener noreferrer').html_safe, link_start: link_start, link_end: '</a>'.html_safe }

4. Verify that the links for Mailgun and Snowplow now have rel="noopener noreferrer". 🎉

MR acceptance checklist

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