Insert block menu (#297)

* Insert block behavior

* Functional with horizontal rule

* Add list and image upload working

* Cleanup typing

* Closest to correct behavior so far

* Improve block insert on list

* Hide (+) after clicking menu item

* Bad merge

frontend/components/Layout/components/SidebarCollections.js

@@ -95,7 +95,7 @@ type Props = {
               <CollectionMenu
                 history={history}
                 collection={collection}
-                onShow={() => (this.menuOpen = true)}
+                onOpen={() => (this.menuOpen = true)}
                 onClose={() => (this.menuOpen = false)}
                 onImport={this.handleImport}
                 open={this.menuOpen}