Consolidate shared issuable spec code

What does this MR do?

Reduces duplicate code by using shared /issuable/new.rb for both issues and merge requests since they're using the same selectors.

This is prep work for the new milestone tests.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content