Collaboration is no longer possible on unpublished posts

I’ve drafted a fix for this: https://github.com/xwiki-contrib/application-blog/pull/135

What changed

• New sharedUsers and sharedGroups properties on Blog.BlogPostClass, shown in the post edit form. While a post is unpublished or hidden, its view lock now allows the creator plus these users and groups.
• New draftSharedUsers and draftSharedGroups blog settings on Blog.BlogClass. A new post starts with these values, and its author can change them. Empty lists keep today’s behaviour: only the author (and admins) can view a draft.
• The Publish button of a draft is shown to every user who can edit it, not only to its creator. Only the creator, the users and groups it is shared with, and admins can view a draft.

Please note: Sharing grants view only. Whether a shared user can edit or publish still depends on their rights on the blog. This avoids an editor of a draft handing out edit rights they could not grant otherwise.

• The draft lock is a single XWikiRights object with the creator as first user. DefaultBlogVisibilityUpdater recognizes it by that and rewrites it only when the share list changes. Other single-user view rights (set by hand, or the per-editor locks of older versions) are left alone while the post is a draft, and removed on publication as in BLOG-172.
• The blog default is copied into the post when it is created (in Blog.BlogPostSheet), so changing the blog settings does not change the existing drafts. It is not copied again when coming back from preview.
• The hide/show button of a published post stays creator-only.

Intentionally excluded: Blog listings and the Unpublished panel still show only the drafts of the current user (doc.creator). Shared users open a draft from a link, which is the BLOG-173 use case. Listing shared drafts can be a follow-up issue.