Change copy and avatar for recurring copilot notifications

[fnisen]

Would be a good to indicate that a project is going unclaimed to make it easier to differentiate from one that is new, so that copilots know whether they've already seen it. Can we change the copy to:
"We're still looking for a copilot for a reviewed project. Please check it out and claim it." And use the Coder error avatar (not sure if it's possible to use different avatars for the same Slackbot)....

[parthshah]

Please overwrite the default avatar with : https://emoji.slack-edge.com/T03R80JP7/coder-error/cd2633216e7fd385.png

[fnisen]

Right, just to clarify all the avatars, related to my comment on this issue #62 (comment)

• When a project is first posted to copilots, use this one: https://emoji.slack-edge.com/T03R80JP7/coder-the-bot/85ae574c0c7063ef.png (if you want, this can also be the default for manager notifications, too)
• When a project is unclaimed and we repost to copilots, use this one: https://emoji.slack-edge.com/T03R80JP7/coder-error/cd2633216e7fd385.png
• When a project is claimed, use this one: https://emoji.slack-edge.com/T03R80JP7/coder-grinning/a3b7f3fe9e838377.png

Thank you.