Downloads page revamp

[caseyhans]

Prettify the assessment downloads page.

• Use card columns for more horizontal real estate
• Let users choose file type
• Adds simple help text for all downloads, based off endpoint help text
• Update/add correlating endpoint help text
• Move a previous css style fix so it applies to PRIME flavor

Before:

After:

Downloads without data are now shown as disabled instead of completely hidden.

[shapiromatron]

Looks great. I made the "no data available" more muted and hide the button (but not the data type). I like the changes a lot, plus we have additional functionality for other export types.