[Slack] Disable Link Previews for Common Domains

[will rudebusch]

see Method 2 here: https://www.makeuseof.com/tag/4-ways-remove-disable-link-previews-slack/

I would add "github.com" to the "disable future attachments" list This will hopefully allow the wg_tech channel to be at least 90% human interaction with an occasional guthub bot post, as opposed to the inverse.

Similarly, disabling Instagram, Facebook, and Twitter would clean up the comms and social media channels.
Should probably ask them first.

I think only a Slack admin can do this.