Based off of @mirka's work in #6805.

Closes #5462.

Implements a File block with the following features:

• Editable text link
• Text link can be copy and pasted to create a normal link
• HTML5 download link
• Copy URL button to copy the media file URL to clipboard
• Accepts both Media Library files and external URLs only Media Library files
• Text link can be changed to point to the Attachment Page
• Text link can be changed to open in a new window
• Accepts and uploads a file drag-and-dropped into the editor (or placeholder)
• Transforms to/from Audio and Video blocks
• Download button text is editable
• Download button is optional