Change Google Drive OAuth scope to app folder

[raucao]

The requirements for full access to someone's Drive are too much for an unhosted open-source app. By changing the scope, the possibility of storing and accessing the same bookmarks from other apps will be removed, unfortunately. However, if someone wants fine-grained control over their user data, they shouldn't use Google Drive in the first place in my opinion.

In order for this not to make people's bookmarks inaccessible, we'll have to migrate data from the current "remotestorage" base directory to a new app folder somehow.

I'm not sure if the trouble is worth it, since there can't be that many Drive users in the first place, because the current dev registration only allows 500 users IIRC. So maybe a more manual migration process could be enough.