Allow parent group milestones to be used on releases API

Summary

Right now parent group milestones are not allowed to be used on releases due to potential ambiguity with duplicate milestone names.

The problem is:

1. we can have this stucture: supergroup -> group -> project
2. both supergroup and group can have milestone with the same name, e.g. 15.6
3. API uses milestone.name instead of id
4. so when user passes 15.6 to the API it's ambiguous which one of the milestones to use

We should address this to allow parent group milestones to be used in releases

Proposed fix

Instead of using milestone names in the API, introduce the ability to use a milestone ID when creating a release. Then update the create release form and milestone combobox to use IDs in the dropdown