Implement an API for deleting accounts/WebIDs.

• (as mentioned by @csarven). The Export Account functionality should be brought to the user's attention when they start the process to delete their account. Emphasize on the point that "once you delete your account, you will lose this and this and this.. and will never get it back. We don't have archives..."
• If the user is exporting their data prior to permanently deleting their account (as opposed to just moving somewhere else), should the server support the HTTP 410 Gone status code?
• Implementers should strongly consider supporting some 2-factor confirmation mechanism prior to deleting the account. (See the original deletion confirmation discussion)
• get consensus on the account deletion API (including suggested 2-factor confirmation mechanisms)
• implement on ldnode - see Implement API for deleting accounts nodeSolidServer/node-solid-server#177
• write docs/examples
• add a 'Delete Account' button to the profile-editor app (which will start the deletion confirmation process)

(Moved from solid/solid-spec#67)