Conflict resolution - Add change history for ssh keys

[therealpaulgg]

To assist with resolving merge conflicts and to make syncing a more seamless experience, we should begin tracking changes made to the ssh-sync key database.

For example:

If a key is deleted, and a user then has a 'ssh-sync sync' command run, the system should be able to detect that most recently a key has been deleted, and delete it from the user's local file system. If there have been more recent changes made on a user's system, then they should take priority over what is on the server.

In the event of a conflict, where the server has deleted and the user has modified, we can either choose to:

1. Do what the server says and replace the file (potential data loss)
2. Do what the user says and ignore the server, potentially causing old data to be added back
3. Prompt the user to manually resolve the conflict (similar to a git resolver)