Add Planning category to the contextual navigation at a group level

At a group level:

• Add new Planning category to the contextual navigation.
• Move Milestones to the new Planning category.
• Move Epics to the new Planning category.

Research to support this decision: ux-research#43 (closed)

Order of the new elements

• Planning should appear below Merge Requests.

• Within the Planning category, the ordering should be: Epics and Milestones (and eventually Roadmap).

• Based on the points above, the new structure of the contextual sidebar at a group level will be as follows:

• Overview

  • Details
  • Activity

• Issues

  • List
  • Boards
  • Labels

• Merge Requests

• Planning

  • Epics
  • Milestones
  • Roadmap

• Statistics

  • Contribution Analytics
  • Members

• Settings (if applicable)

cc @victorwu