importer: add Google Drive importer

It seems the Google Photos API (the "Picasaweb API") is broken and doesn't let you fetch all your photos. Details in #874.

But a workaround is to enable Google Drive's setting to mirror Google Photos in your Drive storage. (Go to https://drive.google.com/drive/my-drive then the Gear at the right, Settings > "Create a Google Photos folder" -> [X] Automatically put your Google Photos into a folder in My Drive)

Even if it weren't just for Google Photos, a Google Drive importer would be nice anyway, so let's do it.

/cc @mpl