Alerts dropdown to modal - EE1

What does this MR do?

Change Alerts dropdown to gl-modal. Part 1 of https://gitlab.com/gitlab-org/gitlab-ce/issues/63197

Keep existing button

Add alert

Edit alert

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• 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