Properly separate tree and item editor JavaScript and CSS
Mostly stop polluting the window namespace in tree_editor.js
Two negligible cleanups in item_editor.js
Add a note that Page.cache_key is not used by FeinCMS itself

Refs #492.
Move imports to the top of various files
Do not hardcode the Page in the preview handler

Refs #487.
Relatedpages extension: Reference FEINCMS_DEFAULT_PAGE_MODEL

Refs #487.