Always clean up previously uploaded files on EmFS on application startup

[agriyakhetarpal]

If we decide to revert #85, this will work as a "pseudo" non-persistent storage of sorts – which would affect files but not notebooks. The idea is that we can loop over files matching the supported file formats (CSV/TSV, PNG, JPEG) and delete them ourselves, despite JupyterLite's efforts to keep them stored persistently.

To be on the safe side, so as not to remove any useful files, I think we can also add a breaking change so that all the uploaded files in the EmFS are stored under, say, jupytereverywhere-files/ or jupytereverywhere-storage/ instead of in the root directory.

However, again, this is iff we revert #85 later on.

[peytondmurray]

Closing because #85 will not be reverted; memoryStorageDriver is an acceptable storage solution for now.