Matrix Signal bridge

A puppeting bridge between Matrix and Signal packaged as a YunoHost service. Messages, notifications (and sometimes media) are bridged between a Signal user and a Matrix user. Currently the Matrix user can NOT invite other Matrix user in a bridged Signal room, so only someone with a Signal account can participate to Signal group conversations. The "Mautrix-Signal" bridge consists in a Synapse App Service and relies on postgresql (mysql also possible). Therefore, Synapse for YunoHost should be installed beforehand.