Consolidate issuable sidebar page object code

What does this MR do?

Part 2 of cleaning up issuable code. (Part 1 is here.) This focuses on consolidating shared code from issue/show.rb and merge_request/show.rb into issuable_sidebar.rb.

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