Navigation to Milestones and Iterations impossible without internal issue tracker

Problem

Just as with labels in #21147 (closed), users won't be able to navigate to Milestones or Iterations if their project does not use our Issues. This is a huge problem because Milestones and Iterations are not only used for Issues, but also Merge Requests and related to other features.

Solution

We should follow what we did in #21147 (closed) with Labels and also move Milestones and Iterations into Project information, as these three items are closely related to each other.