Support sharing media to GutenbergKit

[dcalhoun]

What problem does this address?

The ability to share media between apps is a standard feature on iOS and Android. Currently, GutenbergKit does not support this on Android. On iOS, this feature remains supported as the share feature on that platform still relies upon the Aztec editor.

We should implement support for sharing media to GutenbergKit.

What is your proposed solution?

Done is:

• One can share a file with the WordPress/Jetpack mobile app.
• Sharing a file launches the GutenbergKit editor.
• The editor is populated with an appropriate block:
  • Single image → Image
  • Multiple images → Gallery
  • Single video → Video
  • Audio file → Audio
  • Other → File