Add option to allow inviting other users in portal rooms (#77)

This doesn't mean the invited users get bridged, but for some use cases that doesn't matter

config/bridge.go

@@ -50,6 +50,8 @@ type BridgeConfig struct {
 	InviteOwnPuppetForBackfilling bool `yaml:"invite_own_puppet_for_backfilling"`
 	PrivateChatPortalMeta         bool `yaml:"private_chat_portal_meta"`
 
+	AllowUserInvite         bool `yaml:"allow_user_invite"`
+
 	CommandPrefix string `yaml:"command_prefix"`
 
 	Permissions PermissionConfig `yaml:"permissions"`