Create Collections of Links

[erikaheidi]

With a little more structure, we can have multiple lists of links per user. We'll need to create a dashboard section for link collections, and let the default route /username load a list that is marked as the user's default list (which is automatically created when the user registers, so to make this feature totally optional).

The list would be routed as '/{username}/{list}`. A link should be allowed to be in multiple lists.