Clean up the landing page

[surchs]

The tool landing page shows

• a nice clean "upload" button for the .tsv table
• a nice clean "upload" button for the .json data dictionary
• the "preview" textboxes are removed
• all other functionality is unaffected (e.g. .json button being grey until .tsv is provided, next button ...)
• Upon successful upload (it didn't crash), some super simple UI element says "all good" or ✔️

We still show previews for the .tsv and the .json.

• preferred: nice looking table / rendered component
• also good: a non-editable version of the text field we have now

[surchs]

For JSON rendering: https://github.com/leezng/vue-json-pretty
For .tsv rendering: https://bootstrap-vue.org/docs/components/table as before

[surchs]

Not using https://github.com/leezng/vue-json-pretty anymore, because they don't do deep reactivity: leezng/vue-json-pretty#213 and we kinda need that here. So instead we go with a simple prerendered text.

[surchs]

I'm also skipping the "everything is great" notification here because that would need a toast and that's a bit much right now