Remove link unfurl in private message footer

[pyrech]

Since #213, private message send to participants include a social card:

Let's try to configure our Slack messages (participant, sample and spoil) to not unfurl the url in them:

If you'd like to override these defaults on a per-message basis you can pass unfurl_links or unfurl_media while posting that message. unfurl_links applies to text based content, unfurl_media applies to media based content. These flags are mutually exclusive, the unfurl_links flag has no effect on media content.

See https://api.slack.com/reference/messaging/link-unfurling