4.0rc2

Changes from 4.0rc1:

• Add missing icon file causing manage.py collectstatic to fail
• Fix error on deconstructing DocumentChooserBlock for migrations
• Fix failures when using legacy BaseSetting class
• Fix error insertion logic for modal upload forms
• Fix rich text command palette so that items can be selected by mouse
• Improve API for get_preview_template and get_preview_context methods
• Improve documentation for Page model and related mixins
• Fix live preview error banner overlapping with preview mode selector
• Fix pluralisation on "no permission to delete snippets" errors
• Fix console error for breadcrumbs
• Remove JS debug output from page-editor.js