Changes in uploading code needed for compatibility with B2

[iamareebjamal]

Currently, the uploading code depends on anonymous AWS uploads which allows anyone to upload the content in the bucket. This is obviously insecure but very easy to implement, thus used here. But due to security reasons, B2 does not allow anonymous uploads and closest solution to direct from browser upload is signed URL upload. This may be supported by the frontend by making some changes in the backend and passing in generated signed URLs or may require more changes in frontend as well. This needs to be investigated and fixed before the app can be moved to B2