When a dashboard card is the final oauth dest., need to add some validation and create code

[robinwhitmore]

It's likely that devs will configure oauth before dashboard cards are created and so they may decide to use a dashboard card as the final destination before it's created. When a enters a dashboard card as the final dest., we should do the following:

• check to see if dashboard.card.update webhook is configured in the manifest. If not, add it.
• check to see if a dashboard card with the given name exists. If it doesn't ask if we should create the beginning of a dashboard card using that name or if they want to reenter the value (in case they spelled something wrong).