Enhancement: Registering right sidebar panels as extension

Right sidebar panels can now be registered as extensions via our extension registry. They need to be of type `sidebarPanel`.
The benefit of this is that any app can register additional panels to be shown in the right sidebar while previously the
available panels were hardcoded.

https://github.com/owncloud/web/pull/10111
https://github.com/owncloud/web/pull/10152
https://github.com/owncloud/web/pull/10175
