Accept incoming shares in acceptance tests

Incoming shares are no longer automatically added to the file list of
the sharee. Instead, the user now needs to explictly accept the share.

Currently shares can be accepted only from the Notifications app, so it
must be explicitly cloned before installing Nextcloud if it is not found
in the "apps" directory. Note that the development branches are already
built, so there is no need to explicitly build the app.

With the new sharing behaviour the "share a skeleton file with another
user before first login" scenario is no longer valid (as the user will
need to log in to accept the share, so at that point the skeleton is
already created), so it was removed.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>