Overwrite protection

[acdha]

We should handle the case where someone uploads a file which already exists:

• [x] UI for deleting an existing bag and replacing it entirely
• [ ] UI for reconciling files which exist in the remote end but with different hashes: overwrite, skip, do this for all files, etc.
• [ ] UI for the “Do you want to update an existing bag?” scenario where the path is already a bag which someone wants to add files to, requiring us to append or update the manifest

[acdha]

@dbrunton We might want to consider how necessary the various levels of UI in this issue are for 1.0 – perhaps we simply have an overwrite allow/deny toggle and good error reporting for buckets which are configured not to allow deletion and figure a subsequent release could focus on existing storage navigation in #27.