Workflow to transfer ownership

There should be a workflow to make it possible to transfer ownership of Launchpad objects.

Some use cases:

 * create a product, then assign it to a team one belongs to
 * register a branch for a user without a Launchpad account, then give ownership of the branch after the user has registered.
 * register products, and transfer ownership to an upstream developer

The workflow may look something like that:

 1a. The owner can ask another user to take ownership of an object
 1b. Any logged-in user can ask the owner to give ownership of an object.
  2. A confirmation page is presented, explaining that an email will be sent, and providing a text area with a default message and the ability to edit it.
  3. An email is sent, including links 1. to the concerned object 2. to the initiator of the transfer request 3. to a page to accept the transfer 4. to a unique page to refuse the transfer
  4. Links to a "Ownership transfer" page are displayed on the pages of the initiator, the recipient, and the concerned object.
  5. These links point to an ownership transfer page recap'ing the contents of the email.
  6. The pages to accept and refuse the transfer are form with a single button and a text form for an optional explanation.
  7. On posting any of those forms, an email is sent to the transfer initiator, the ownership of the transferred object is changed, and the ownership transfer object is deleted.