Add ability to update tree once it's been shared

[paulgb]

Suggested on Twitter: https://twitter.com/NotScottSiskind/status/1277910627865018373

For this to work, we still need to capture the auth token and have the user enter Treeverse from a twitter.com domain. I suggest this work by allowing the user to add subtrees from a share link to an open Treeverse by pasting the URL into a box or something.

Another approach would be to allow JSON export (#65) and make the Treeverse (in-extension) window a drop target for file uploads, then when the file is dragged in splice in any overlapping nodes (and replace the root if the current root is contained in the tree).