Move required styles for add issueable form

What does this MR do?

This MR moves the required styles for the form used to add linked / related issues from ee to core.

The Feature was introduced in #212329 (closed), but the stylesheet was not moved, yet.

Screenshots (strongly suggested)

see related issues #273558 (closed) and #258678 (closed)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

As the styles are only moved from one scss file in ee to the core file there should not be any impact. The code is already tested for ee version, so the only concern might be the location of the code in the new file.

Related to #258678 (closed), #273558 (closed)