Add "Create subpage" icon to shared pages in letf panel

[MurzNN]

What makes the feature necessary? When I have a page, shared with me by another user, I can add subpages to it via /page command, but not via the + icon on the left bar with the tree of pages, because it's missing!.

How would the feature work? Will check if the current page is shared to me (I'm not the owner) and I have edit permissions - draw the + icon to the page item in the left tree.