Fix high latency autosave bug in editor.new

Closes #4400
- Added focusCursorAtEnd property to codemirror component, to set the cursor at document end on load. Used for posts that have been edited
- centralized codemirror init code in component
Merge pull request #4459 from novaugust/slow-save-errors#4400

Fix high latency autosave bug in editor.new