fix: Add access to document TOC on mobile (#2279)

* Add TOC button for mobile

* Undo NewDocumentMenu changes

* Place the toc button in the correct position.

* Pass menu props to menuitem

* Update app/menus/TableOfContentsMenu.js

Co-authored-by: Tom Moor <tom.moor@gmail.com>

* Update app/menus/TableOfContentsMenu.js

Co-authored-by: Tom Moor <tom.moor@gmail.com>

* Use the existing prop type

* Write menu inside actions prop

* Prevent blank webpage behaviour for toc

* Use href instead of level to determine target

* Update app/scenes/Document/components/Header.js

Co-authored-by: Tom Moor <tom.moor@gmail.com>

* Add heading to menu items

* Use existing Heading component

Co-authored-by: Tom Moor <tom.moor@gmail.com>

app/components/ContextMenu/MenuItem.js

@@ -15,6 +15,7 @@ type Props = {|
   target?: "_blank",
   as?: string | React.ComponentType<*>,
   hide?: () => void,
+  level?: number,
 |};
 
 const MenuItem = ({
@@ -86,6 +87,7 @@ const Spacer = styled.svg`
 export const MenuAnchor = styled.a`
   display: flex;
   margin: 0;
+  margin-left: ${(props) => props.level * 10}px;
   border: 0;
   padding: 12px;
   width: 100%;