Share to connect

[simonpcouch]

Stacks on #404. Just getting this off of my machine—not yet reviewed or tested.

There are two potentially-shared items supported by this PR:

• The conversations themselves
• Conversation artifacts

The artifacts are a good bit easier and I'm less concerned about them. The conversation sharing is a bit gnarly right now, but should get a good bit easier as the shinychat team moves their conversation persistence work forward. Might be worth just removing the conversation sharing part of this PR and waiting a couple weeks for that work to land in shinychat.

[simonpcouch]

• Maybe "Share to Connect" isn't just go to connect--it should instead be "Share with..." and that link goes to a person
• What if Share was instead applied to tool results? Or plots/tables?
• Ideally, 'The relationship between what happened and what was shared is closer'