Custom Metrics backend and UI for creating and editing queries

What does this MR do?

Adds support for custom metrics

Screenshots (if relevant)

When there's no custom metrics

Form for adding metrics

Adding

Editing

When there's custom metrics

When there's custom metrics and no deployments

No default metrics/custom metrics

With no deployment

When deployed and no common metrics were found

The dashboard itself

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Implement Query Validation using Prometheus
• Use added queries to query prometheus and return responses in additional queries endpoint
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

Related to: #2273 (closed)

CE backport of common metric changes:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/17586