Allow reverting of locally stored shared documents

[jessevdk]

Currently when you open a remote document (either from the gallery or using a shared URI), a local copy of that document including its share key are stored in the users indexedDB. The next time the same document is requested, this stored document is opened instead. The reason for this is that this allows you to make local modifications to such document which are automatically stored, while not loosing those same modifications the next time you open the same remote document (or creating a duplicate).

I think this behaviour works well in most cases, but it needs some improvements:

1. It should show whether or not the locally stored document has been modified relative to the remote document (note that remote documents are uniquely identified and can't change, i.e. are immutable under their identifier).
2. Users should be allowed to revert to the original document, or maybe view a diff or something similar.