Convert stylesheets/pages/issuable.scss into page specific bundle

What does this MR do and why?

• Move issuable.scss into page_bundles
• Move sidebar content of issuable.scss into sidebar.scss
• Move .issuable-list styles into new page_bundles/issuable_list.scss

Screenshots or screen recordings

–

How to set up and validate locally

Issuables

• Issue: http://gdk.test:3000/flightjs/Flight/-/issues/3
• MR: http://gdk.test:3000/flightjs/Flight/-/merge_requests/1
• Epic: http://gdk.test:3000/groups/flightjs/-/epics/4

Issuable List

• Issues: http://gdk.test:3000/flightjs/Flight/-/issues
• MRs: http://gdk.test:3000/flightjs/Flight/-/merge_requests
• Epics: http://gdk.test:3000/groups/flightjs/-/epics

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #239838 (closed)