Add hook for inserting site-specific CSS/JS into the editor interface

This change is essentially a roundabout way of eliminating RCA-specific styling from rich-text.scss, meaning that h2 elements no longer have a horizontal rule above them, and images appear with their correct alignment within the rich text editor again.

More specifically:

 * Tidy up the proliferation of tag libraries in wagtailadmin/templatetags - most of them are now merged into a single wagtailadmin_tags library
 * Add a new template tag `hook_output` to insert content returned from hook functions onto the page
 * Use this to define two new hooks, `insert_editor_css` and `insert_editor_js`, through which apps can specify CSS/JS tags to insert into the editor
 * Replace the RCA-specific styles in rich-text.scss with generic ones, with the intention that RCA will now override them with custom CSS passed in via `insert_editor_css`.