Allow updating app instance's name and icon (and description?)

[tbroyer]

Currently, an instance's name, description and icon are provided by the Portal at instantiation time (as a partial copy from the application), and cannot be changed afterwards.

Given that those values are displayed on the authorization page, they need to be updateable and customizable on a per-instance basis.

See ozwillo/ozwillo-issues#2

[tbroyer]

An instance should also be able to update its own destruction_uri, destruction_secret, status_changed_uri, status_changed_secret, needed_scopes, etc.

Updating the URIs might be needed for migrating an application to a new name (new domain name)