Store the draft release on the client side

Extracted from the Redirect back to "new release" page after creat... (#296084 - closed)

See the video: https://www.youtube.com/watch?v=tZIPG0eRTwU

We want the following workflow to work:

1. user starts creating release
2. clicks on the create milestone button
3. creates the milestone and gets redirected to the new release page.
4. continues to edit the release

Proposal

Update the New Release form to restore unsaved changes, like the New Issues page does. This way, when we redirect back to this page after creating a new milestone, previously-entered data isn't lost.