WIP: Move Charts Alerts form to modal

TODO

• move loading spinner to button
• move error message to form
• fix form layout
• add label to dropdown
• add dropdown
• close chart tooltips when modal opens

What does this MR do?

Move charts' Alerts Editor from fake dropdown to a real modal

Part 1 of https://gitlab.com/gitlab-org/gitlab-ce/issues/63197

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers