Add link folders.

This pull request allows links to be sorted into folders and subfolders.

* Links are added to a folder by drag and drop.
* Folders belong to a particular user; users won't see the same folders
even if they have the same links. This makes sense because an author
will want to organize the links they create very differently from the
editor who vests them. (If we eventually let editors at the same
organization see each other's links, we can also let them see each
other's folders.)
* The same folders show up under "Links" and "Vested Links." Query
whether "Vested Links" still makes sense -- we could just have "Links"
show links either created or vested by the user.
* If links are paginated, the folders show up on every page of results.
* Folders can only be deleted if they are empty.