Improve redux-undo, so that when an undo/redo happens, the `lastSyncAt` timestamp increases

[munen]

Related: #60

This will eliminate the UX isue that when undoing/redoing 'quickly', the 'security popup' comes asking the user if she she wants to pull/override to/from the server or cancel the sync. An undo/redo should always persist to the server, because that's what the user wanted.

Resources:

• Optional implementation on top of redux-undo: [#237] Allow filtering state properties before save it to history omnidan/redux-undo#238
• Alternatively, implement custom logic in actions/org.js. Potential algorithm: Whenever there is a lastSyncAt in state.org.future, then we're in the past. Hence, ignore the lastSyncAt date.

Alternative

By default, don't let the user decide on a conflict, but overwrite the remote file

Rationale:

• For non-programmers, it's confusing to have the option to chose between pull, overwrite and cancel
• "Last wins" is the common way to resolve conflicts in 'non-programmer' applications.
• organice saves an .organice-bak file on initial download of an Org file for the current session