This issue is to discuss possible ways to enable portability of user data.

(Originally opened by @nicola)
(Moved from solid/solid-spec#72)

Portability specifically means that "the user can take their data elsewhere", whenever they wish. It is a combination of the following features:

• A user can export (download) all the data in their account (see #13 - API to download/export account data
• A user can move all the data to another Pod/account (see #12 - Enable users to migrate/copy their accounts to another server
• Optionally (but recommended), the old server should maintain 301 Redirects to the new location.