Conflict detection and resolution

[sendhil]

When a post/page has been updated both in the server and the app, we don't do any conflict resolution and the changes in the app can get overwritten by the revision on the server
We should (easiest first):
Always give priority to the version in the app (changes overwritten in the server can be restored through revisions)
Alert the user of the conflict and offer a choice
Ideally allow the user to review the changes, but that seems like a complex UI for mobile

[koke]

There's a good start in #10. Related: #147

[sendhil]

Moving to 4.1.

[sendhil]

Moving to 4.2.

[sendhil]

Moving to 4.4.

[astralbodies]

Closing issue and migrating to new repo.