File sharing ui update

[sumanyu]

Major changes

• Update UI to match @KellyLi design
• Users can now drag and drop upload files
• Users can now delete files by clicking x
• Added auxiliary fns to help with S3 operations

  To Do

• Add spinner and progress dialog when uploading and deleting files

[davinwong]

nice. the only nice-to-have comments i would make are:

• some sort of signal for loading so it does not become confusing when waiting for file upload
• when clicking X on a file, the tile should instantly disappear even though the file is not yet gone.

good to merge

[davinwong]

fixed a merge conflict