Resolve "Add additional aliases for reviewer quick commands"

What does this MR do?

This adds /reviewer and /remove_reviewer commands as aliases to /assign_reviewer and /unassign_reviewer for more discoverability, as well as refactors a lot of specs to make it possible (and easier) to test future aliases.

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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Closes #297011 (closed)