Share URL for posts and comments

[jhass]

There should be a share URL attribute akin to for example the html_url property in the Github API.

Clients that want to produce a sharable URL to some content should not have to guess this according to whatever the route on the server currently is, even though these proved to be fairly stable over the years.

We might want to return this only for public content to nudge clients away from producing share messages for private content.