Add support for room shares to the share manager

Room shares are implemented in an external app (Nextcloud Talk), so in
order to keep the share manager as isolated as possible from room share
specifics all the validity checks are done in the provider of room
shares. However, due to the code structure it is necessary to explicitly
check for room shares in "generalCreateChecks" to prevent an exception
from being thrown.

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