Set external_url on messages sent to Matrix

[psrpinto]

This PR sets the external_url property on the matrix event, as per the spec:

Application services should include an external_url in the content of events it emits to indicate where the message came from. This typically applies to application services that bridge other networks into Matrix, such as IRC, where an HTTP URL may be available to reference.

The external_url property is set for both "normal" messages and messages in a thread:

• https://${team.domain}.slack.com/archives/${slackRoomId}/p${slackEventTs}
• https://${team.domain}.slack.com/archives/${slackRoomId}/p${slackEventTs}?thread_ts=${slackThreadTs}

[psrpinto]

Tested locally and seems functional, so merging.