Provide import/export bookmarks functionality on mobile

There are users asking after importing/exporting bookmarks (for instance on our blog: https://blog.torproject.org/comment/283638#comment-283638). Right now it seems only Sync is available which we have disabled, so we should think about a different solution or revise our Sync treatment.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information