Web interface to load data into local Xena hub

In order to visualize their private datasets, users of UCSC Xena browser have to upload data using separate desktop application. This behavior does not fit preferred workflow, since all visualizations are carried out in the browser and using desktop client only for uploading data is confusing for some users.

The goal of this project is to move existing data upload functionality from desktop client to the browser. In addition to that, import module would be improved with extensive error handling and uniformed user experience overall.