refs: https://github.com/TryGhost/Team/issues/3139https://github.com/TryGhost/Team/issues/3140
- Added duplicate post functionality to post list context menu
- Currently only a single post can be duplicated at a time
- Currently only enabled via the `Making it rain` flag
- Added admin API endpoint to copy a post - `POST ghost/api/admin/posts/<post_id>/copy/`
- Added admin API endpoint to copy a page - `POST ghost/api/admin/pages/<page_id>/copy/`
refs https://github.com/TryGhost/Toolbox/issues/363
- this API framework is standalone and should be pulled out into a
separate package so we can define its boundaries more clearly, and
promote better testing of smaller parts