Frontend for Cross-project environments dashboard MVC

What does this MR do?

Master MR to complete all necessary work for the environments dashboard.

• Extract components shared between pipeline (operations) dashboard and environment dashboard. !10348 (merged)
• Set up route + page behind feature flag for new dashboard.
• Construct dashboard behind feature flag.

What are the relevant issue numbers?

#3713 (closed) SCSS Backport to CE at gitlab-org/gitlab-ce!27498

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer