Render markdown

[meissadia]

Closes #421

Changes

• Adds react-markdown library
• Successfully renders markdown from Submission's validation description
• Renders a more comprehensive markdown test at route /markdown (must be signed-in).

How to test this PR

1. Upload a file for 123456789TESTBANK123 that has warnings (sbl-validations-pass.csv)
2. Visit http://localhost:8899/markdown
3. Observe the markdown from the Validation's description is rendered as a list item

Notes

• We would need to add a plugin to add support for advanced elements like Tables or Strikethrough, but I've followed up with the Backend team to understand which Markdown elements we should expect from the API and only basic (list, bold) are expected to be used, so we're good to go with the current basic configuration.

Screenshots

[billhimmelsbach]

This looks good @meissadia! I'm going to let @shindigira get a peek at this one before I approve: maybe just message him and make sure he's ok merging this branch into his errors and warnings work over here, either now or when it hits main: #423

[shindigira]

Thanks for this!