Add link folders.

This commit 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.
Folder tweaks.

Fix display of link when one user has it in a folder and other doesn't;
tweak folder display.
closes #344
Merge branch 'messages' into develop
closes #286
Merge branch 'dev.folders' of https://github.com/jcushman/perma into jcushman-dev.folders

Conflicts:
	perma_web/perma/urls.py
	perma_web/perma/views/user_management.py
closes #346
Merge branch 'develop' of github.com:harvard-lil/perma into develop
Add folder checkboxes.
Remove ipdb comment.
Merge pull request #348 from jcushman/dev.folders2

Add folder checkboxes and move selected items button.
Merge branch 'develop' of github.com:harvard-lil/perma into develop
closes #356
closes #355
closes #355 and closes #354
paging and sorting across pagination and sorting buttons for users. fixes #328
Merge branch 'develop' of github.com:harvard-lil/perma into develop
fixes #351