Improve CMS post summary UI

[ryon]

See SlateFoundation/slate#235 (comment)

CMS editor improvements

• Move post summary from override to main app
• Add toolbar button to toggle post summary (default off)
• Show blog post summaries by default in skeleton
• Improve some CMS editor emptyText
• Discourage empty posts in CMS editor
  • Include a Markdown content item by default
  • Cull empty Markdown composers when saving
  • Warn if post being saved has no content blocks

Minor skeleton upgrades

• Use a system font stack instead of Helvetica
• Include a placeholder logo instead of linking out

[themightychris]

@ryon you probably want to use the beforetoggle and toggle events rather than click events: https://docs.sencha.com/extjs/6.2.0/classic/Ext.button.Button.html#event-beforetoggle

[ryon]

@ryon you probably want to use the beforetoggle and toggle events

@themightychris I’m using toggle, which menucheckitem doesn’t have, for whatever reason