Bookmarking public pages

Users should be able to bookmark others peoples' public pages and revisit them later.

(Probably not ready to work on yet -- this feature might be redundant with some Collection features coming up and/or just a default, private "Bookmarks" collection for each user)

The issue does not name files, tests, or an entry point to inspect. First resolve whether bookmarking belongs in the upcoming Collection features or a private default Bookmarks collection; implementation should wait until that product direction and the expected revisit behavior are defined.

Written by the indexing model from the issue text.