Refactor Milestone select sidebar component to Vue+Apollo

Currently, issue sidebar Milestone select component is implemented with HAML and jQuery. In order to align all sidebar components to use the same stack, make them reusable and be ready for GraphQL subscription-based realtime updates, we need to refactor this component to use Vue + Apollo client stack.

Availability and Testing

Manual job package-and-qa will need to be run in the MR that introduces the change.

Ensure test coverage in unit/integration tests