Implement Auto-save functionality

[cb-fumix]

In order to lose progress when writing articles, content should always be saved in the background.

• Every post should have an autosaved shadow copy that is constantly saved alongside the currently manually saved version (if any)
• Shadow copies are always removed when a user manually saves the post.
• If an autosaved shadow copy is available and diverges from the currently persisted main post, a user has the option to restore the shadow copy and continue working with it.
• Autosaved shadow copies are never publicly or otherwise viewable, unless they are restored and then manually saved.
• There can only be one autosave per post, concurrent usage can overwrite any previously autosaved copies.
• Change "draft" checkbox and "cancel/save" button to "Cancel", "Save as draft", "Publish" -> "Cancel" leaves the edit window, autosaving. "Save as draft" saves a post as a draft and removes any shadow copy. "Publish" saves and publishes (draft=false) post and also removes any shadow copy.