Follow up of #202
#256 introduced the cross contract calls in execute.

When creating proposals, we need to let users add a contract for outcomes.

The minimum is to have an input for the contract address (validating that the contract does exist, could also be done in the contract actually) and some instructions, link to a docs to know what to do.

What would be better is to have a dropdown with some templates to download. We don't want to get into the rabbit hole of having a web IDE and deploy from the app. That still needs to be done by the user. We just can give some helpers.