Fix tutorial toggle clearing file selector

[mccalluc]

• Fix Toggling tutorial clears file selection #838
• and fix a similar problem with the maximum rows: Need to be sure that input elements aren't in a container with any reactive dependencies.
• fix sample csv in cloud #810, and drop the columns-only UI. Not necessarily coupled, but getting rid of one option makes this easier to manage.
• "sample" to "demo": We also have code samples and statistical samples, so it seems like a new name would be more clear?
• If running in sample mode, just show a warning to explain why upload is not allowed.