Allow Promoting Content from Unity Editor

[allister-beamable]

Reason for the Feature

A customer recently asked me whether you could promote content from realm to realm from within Unity, and it occurred to me that it would be reasonable to do so. The API for it would be the same as what Portal uses for realm-to-realm promotion, and admins in Unity Editor would have the right degree of access.

Proposed Solution/Requirements

Add a menu item or button, wherever seems most logical, for promoting content from within Unity Editor.

Alternatives or Workarounds

It is already possible to promote content from Portal, so this is purely a nice-to-have feature.

[cdhanna]

The CLI side of this is: #4393

[cdhanna]

we need to think carefully about the UX inside the Unity Editor, so that people understand,

1. it is NOT local to remote, but actually remote A to remote B.
2. what realm choices they have
3. where the promote button is located