Move chart alerts into dropdown menu

Problem to solve

We are hoping to add a number of additional features to charts (for example https://gitlab.com/gitlab-org/gitlab-ce/issues/60621), all of which will require adding a dropdown to the charts on our metrics dashboard.

Since this dropdown will live in the same place as alerts do currently, we need to:

1. Add the dropdown
2. Move the existing alerts functionality into it

Intended users

Sasha the Software Developer
Devon the DevOps Engineer
Sidney the Systems Administrator

Further details

This work supports the Incident Management Vision

Design

Introduce the dropdown menu and move alerts into it:

Introduce a modal to contain the existing alerts functionality:

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

Links / references