Do not add system message for self joined users to file rooms
Do not add system message for self joined users to file rooms

When a user with access to a file joins its file room the user is added
as a persistent participant, so a "XXX joined the conversation" system
message is shown. However, if a user does not have direct access to a
file and joins its room the user is not added as a persistent
participant, so the system message should not be shown in that case.

Signed-off-by: Daniel Calviño Sánchez <bd7629e975cc0e00ea7cda4ab6a120f2ee42ac1b@gmail.com>

nextcloud/spreed,nextcloud/spreed,nextcloud/spreed