Update settings nav items to reference work items

Merge Requests

1. frontend settings issues -> work_items references update: Update group/project settings nav items to refe... (!214206 - merged)

2. backend group settings -> work_items route: Update route for group work item settings (!214399 - merged)

Now that issues and epics have become work items - the settings pages need to be updated.

Group level settings

• Update the nav item for Settings -> Issues to become Settings -> Work items
• Update the page title from Issues to Work Items

Project level settings

Under Settings -> General -> Visibility, project features, permissions:

• Rename 'Issues' to 'Work items'
• Update the description text:
  • old: Flexible tool to collaboratively develop ideas and plan work in this project. Learn more.
  • new: Plan and track work with flexible objects and views. Learn more.
• Update the 'learn more' link to be https://docs.gitlab.com/user/work_items/