Use issuable_list page specific styles on service desk

What does this MR do and why?

Describe in detail what your merge request does and why.

Follows on from !106357 (merged)

Service desk also has the same issues

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

• Service desk set up is pretty hard.
• Instructions to test locally are being added as part of gitlab-development-kit!2827 (diffs)
• One thing to note, if you use an existing email address it will mark all your emails in 'inbox' as read.

@leetickett-gitlab

To quickly access a list of service desk issues in GDK:

1. Update /gitlab/config/gitlab.yml
   1. Find incoming_email: and set enabled: true
   2. Find service_desk_email and set enabled: true
2. gdk restart
3. rails c

   i = Issue.last
   i.external_author = "gitlab@example.com"
   i.author = User.find_by(username: "support-bot")
   i.save

Alternatively, we could probably just create a new service desk issue using FactoryBoy (again, from a rails console):

FactoryBot.create(:issue, project: Project.find(6), author: User.find_by(username: "support-bot"), service_desk_reply_to: 'service.desk@example.com')

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.