Basic previewers

[lmarini]

Implement new basic previewers

### Tasks
- [ ] better text previewer (current one shows all the text in one narrow column)
- [ ] csv (ability to sort?)
- [ ] json
- [ ] pdf
- [ ] geospatial
- [ ] markdowns

[ddey2]

Discussion from Jun 20 dev meeting:

1. We can have a expand button to expand to the entire page
2. use pdf.js
3. Use 'Show more' to may be show '500' chars instead of showing the entire text.
4. Keep the respective dependencies in each previewer bundle
5. think about previwers layout if multiple visualizations are avaialble