Add a "bridge" mode that allows any users to chat in specific groups.

As the title says, the "bridge" mode allows not only admins but also normal users to chat in specific groups. When linking a chat to a group, add an option for "bridge".

Also, is it difficult to add a "sender indicator" from master to slave in a less hacky way? As far as I know, EFB doesn't seems to support the "sender" field, and we could only achieve it with manually prefixing a message.