lakeFS webui File Viewer

[eladlachmi]

Add a file viewer to the lakeFS webui with an extendable list of renderers for supporting different file types.
Initially, markdown and Jupyter notebooks will be supported, but additional renderers can be added.

For choosing the correct renderer for a file we use the following process:

1. If the file's metadata includes a "Content-Type", use that
2. If the file has a supported file extension, use that
3. The file cannot be displayed, but download and copy-to-clipboard buttons will still be available