File Dropzone: Make message customizable

[austinoneil]

Description

Make filezone messages customizable. Also fixed a minor issue with a "null" class rendering.

Resolves #1741

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How Has This Been Tested?

Added unit tests.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings

[netlify]

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Lighthouse	1 paths audited Performance: 26 (🟢 up 2 from production) Accessibility: 98 (no change from production) Best Practices: 92 (no change from production) SEO: 92 (no change from production) PWA: - View the detailed breakdown and full score reports

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[coliff]

Works good! https://deploy-preview-2388--moduswebcomponents.netlify.app/?path=/story/components-file-dropzone--default&args=instructions:Upload+Kitten+Pictures 👍