WIP: MVC of SLI Alerts

What does this MR do?

Allows adding, editing and removing alerts.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

create an alert	update and delete an alert

HTML email	Plain text email

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• 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
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #5158 (closed)