Add support to upload a .excalidraw file to create a new page

[macintushar]

Excalidraw allows users to export their drawings as a .excalidraw file. Support needs to be added to upload this file and create a new page based on the data in that file.

[vanyatwo]

for now - you can create a blank page and click "Open" in a menu, select your file and import it :)