Redirect back to "new release" page after creating milestone

Summary

When creating a new release, if you create a new milestone within the workflow - the milestone is created, but the release is not.

Steps to reproduce

1. Go to project overview->releases
2. Create a new release
3. Fill in all the details until you reach milestone
4. Create a new milestone
5. Fill in all the details for the milestone and complete the creation
6. Go back to releases

What is the current bug behavior?

Creating a new milestone kicks you out of the create release flow

What is the expected correct behavior?

A milestone is created within the release creation (similar to creating a new environment in the feature flag workflow) or at least after milestone creation, return to the create release form with all the previous details filled out

I created a video that displays the workflow: https://www.youtube.com/watch?v=tZIPG0eRTwU

Proposal

Update the New Milestone page to allow the behavior of the Create Milestone button to be configurable:

• By default, clicking it would navigate to the new milestone's detail page (as it does today).
• However, if a back_url URL parameter is provided when navigating the Create Milestone page, the user will be redirected to the provided URL instead.

Also implement #372716 (closed)