Specify cloud-init user-data via `textarea`

[sudomateo]

In #800, a file upload input was added to receive a user's cloud-init user-data. This issue requests improving this flow with a textarea so that users can paste in user-data without needing to create and upload a file. This was briefly discussed in #800 (comment).

One could argue both for and against this feature, as adding it adds complexity and keeping it out adds friction, but the major cloud providers (e.g., AWS, GCP, Azure, DigitalOcean) all have the ability to specify user-data via a textarea. It would be nice for user consistency if we did.