Support sharing of lists

[heralden]

There should aleady be web services for this.

We'll want a button on the results page beside the list title, and a button for the list row on the lists page, for sharing a list with a user. It should open a modal where you can input the email of a user to share with, or modify existing users the list is already shared with. The icon for the button should probably change when a list is actively shared.

If we add this, we'll likely also need to add a new panel to the profile page where a user can see all the lists they're currently sharing and with who, and be able to edit each of these.