Fill channel.external_url field in m.bridge state key

[MurzNN]

In MSC2346 there is exists channel.external_url field in m.bridge state key, but mx-puppet-slack didn't fill it.

In mx-puppet-bridge it is defined, and in mx-puppet-slack it is generated here https://github.com/Sorunome/mx-puppet-slack/blob/master/src/slack.ts#L153

So seems simply mx-puppet-slack didn't transfer it to mx-puppet-bridge?