Ability to create milestones on the fly

Description

We'd like to be able to create milestones on the fly from the issue creation form, very much like gitlab-ce#12574, but for milestones of the respective project.

As this seems very similar to gitlab-ce#12574, the proposal very much sounds the same:

I click the New issue button, fill up fields Title, Description, Assignee etc.

Then I want to select a milestone that doesn't exist yet, but I can't do it staying on the page.

I have to save the issue without a milestone, then go to the Milestones section, create a new milestone, go back to edit the issue I've just created, select the new milestone and save the ticket.

Proposal

Add an ability to create milestones for the project on the fly from the issue creation page.

Links / references

gitlab-ce#12574

Documentation blurb

Overview

When creating an issue for a no-yet-existing milestone, allows a user to create a milestone on-the-fly directly from the issue creation page without going the detoour via the Milestone page.

Use cases

Create a milestone while creating an issue.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.