Re-add "public portals" configuration option #610
Description
In the Python version, it was possible to set "bridge.public_portals" to true. This enabled Signal groups with an unrestricted invite link and public Matrix chats to be bridged in relay mode.
It seems like this feature might be planned based on a comment in the code.
As an admin of a public Matrix room that also wants a presence on Signal, this seems like a nice option - at the moment, the bridge bot keep changing the join rules on Matrix side every now and then. I started working on a PR, but I'm not sure how to best expose the configuration to inviteLinkToJoinRule.