File Upload functionality

Hush-NG Conversations will have the ability to upload arbitrary data files, first limited to about 26KB, which will fit in a single transaction.

The user should be able select any file on their computer via standard file dialog and/or drag and drop.
We should calculate the file size and display in the UI.
For now, if the file is larger than 53*511=27083 bytes, show an error div about it being too large
Bonus: Compute the SHA512 of the file, which Hush-NG will need, an optionally show to the user

MyHush / hush-ng