[feature](dapp): import VCs from uploaded JSON files

[pseudobun]

Summary

Currently users can import VCs by pasting the JSON object into the designated field. When dealing with multiple VCs and backing them up/saving them/exporting them, saving them in a file will prove useful. By giving users the ability to upload .json files containing VC/s, they could back them up in files and store them wherever they want. Additionally, the code added for this could be useful with #309, when providing support for Google Drive as a storage solution.

Useful resources: #318 already added a UploadButton component.

Expected behavior / User Story

When importing VC/s, users should have another button for uploading files instead of pasting the JSON text to designated field.

[tadejpodrekar]

Before this we first have to add support for batch operations #148.