Validate: import data

When importing data, validate that the input JSON file is inside an array container. When it's not, show an error screen. Currently, there is no response being shown to the user.

The code for this functionality resides in the `importer` repo, but we primarily use it inside the dashboard.

We currently show it as a help message but it's hidden behind the help icon.


Start with the import flow in the dashboard and the related functionality in the `importer` repo. Read how invalid JSON input is currently surfaced as a help message, then verify the behavior for a file whose top-level value is not an array. Done means that case displays an error screen rather than leaving the user without a visible response.

Written by the indexing model from the issue text.