Prevent unfurling links in the "New coordinated pipeline" Slack message

What does this MR do?

Since the link to the branch commits requires the user to login, there is no point in unfurling it since it looks like this:

This MR changes the default for the unfurl_links and unfurl_media settings to false (it was not being set before). A new additional_options parameter has been introduced to Slack::Message.post. unfurl_links and unfurl_media can be passed in the additional_options hash to override the default value of false.

Since the Slack::Message.post method already has 5 parameters (Rubocop complains with more than 5 parameters), the thread_ts parameter has been removed. thread_ts can be passed as an additional_option instead.

Author Check-list

• Has documentation been updated?